![Chris Down](/assets/img/avatar_default.png)
cgroup v2 introduces two memory protection thresholds: memory.low (best-effort) and memory.min (hard protection). While they generally do what they say on the tin, there is a limitation in their implementation that makes them difficult to use effectively: that cliff behaviour often manifests when they become eligible for reclaim. This patch implements more intuitive and usable behaviour, where we gradually mount more reclaim pressure as cgroups further and further exceed their protection thresholds. This cliff edge behaviour happens because we only choose whether or not to reclaim based on whether the memcg is within its protection limits (see the use of mem_cgroup_protected in shrink_node), but we don't vary our reclaim behaviour based on this information. Imagine the following timeline, with the numbers the lruvec size in this zone: 1. memory.low=1000000, memory.current=999999. 0 pages may be scanned. 2. memory.low=1000000, memory.current=1000000. 0 pages may be scanned. 3. memory.low=1000000, memory.current=1000001. 1000001* pages may be scanned. (?!) * Of course, we won't usually scan all available pages in the zone even without this patch because of scan control priority, over-reclaim protection, etc. However, as shown by the tests at the end, these techniques don't sufficiently throttle such an extreme change in input, so cliff-like behaviour isn't really averted by their existence alone. Here's an example of how this plays out in practice. At Facebook, we are trying to protect various workloads from "system" software, like configuration management tools, metric collectors, etc (see this[0] case study). In order to find a suitable memory.low value, we start by determining the expected memory range within which the workload will be comfortable operating. This isn't an exact science -- memory usage deemed "comfortable" will vary over time due to user behaviour, differences in composition of work, etc, etc. As such we need to ballpark memory.low, but doing this is currently problematic: 1. If we end up setting it too low for the workload, it won't have *any* effect (see discussion above). The group will receive the full weight of reclaim and won't have any priority while competing with the less important system software, as if we had no memory.low configured at all. 2. Because of this behaviour, we end up erring on the side of setting it too high, such that the comfort range is reliably covered. However, protected memory is completely unavailable to the rest of the system, so we might cause undue memory and IO pressure there when we *know* we have some elasticity in the workload. 3. Even if we get the value totally right, smack in the middle of the comfort zone, we get extreme jumps between no pressure and full pressure that cause unpredictable pressure spikes in the workload due to the current binary reclaim behaviour. With this patch, we can set it to our ballpark estimation without too much worry. Any undesirable behaviour, such as too much or too little reclaim pressure on the workload or system will be proportional to how far our estimation is off. This means we can set memory.low much more conservatively and thus waste less resources *without* the risk of the workload falling off a cliff if we overshoot. As a more abstract technical description, this unintuitive behaviour results in having to give high-priority workloads a large protection buffer on top of their expected usage to function reliably, as otherwise we have abrupt periods of dramatically increased memory pressure which hamper performance. Having to set these thresholds so high wastes resources and generally works against the principle of work conservation. In addition, having proportional memory reclaim behaviour has other benefits. Most notably, before this patch it's basically mandatory to set memory.low to a higher than desirable value because otherwise as soon as you exceed memory.low, all protection is lost, and all pages are eligible to scan again. By contrast, having a gradual ramp in reclaim pressure means that you now still get some protection when thresholds are exceeded, which means that one can now be more comfortable setting memory.low to lower values without worrying that all protection will be lost. This is important because workingset size is really hard to know exactly, especially with variable workloads, so at least getting *some* protection if your workingset size grows larger than you expect increases user confidence in setting memory.low without a huge buffer on top being needed. Thanks a lot to Johannes Weiner and Tejun Heo for their advice and assistance in thinking about how to make this work better. In testing these changes, I intended to verify that: 1. Changes in page scanning become gradual and proportional instead of binary. To test this, I experimented stepping further and further down memory.low protection on a workload that floats around 19G workingset when under memory.low protection, watching page scan rates for the workload cgroup: +------------+-----------------+--------------------+--------------+ | memory.low | test (pgscan/s) | control (pgscan/s) | % of control | +------------+-----------------+--------------------+--------------+ | 21G | 0 | 0 | N/A | | 17G | 867 | 3799 | 23% | | 12G | 1203 | 3543 | 34% | | 8G | 2534 | 3979 | 64% | | 4G | 3980 | 4147 | 96% | | 0 | 3799 | 3980 | 95% | +------------+-----------------+--------------------+--------------+ As you can see, the test kernel (with a kernel containing this patch) ramps up page scanning significantly more gradually than the control kernel (without this patch). 2. More gradual ramp up in reclaim aggression doesn't result in premature OOMs. To test this, I wrote a script that slowly increments the number of pages held by stress(1)'s --vm-keep mode until a production system entered severe overall memory contention. This script runs in a highly protected slice taking up the majority of available system memory. Watching vmstat revealed that page scanning continued essentially nominally between test and control, without causing forward reclaim progress to become arrested. [0]: https://facebookmicrosites.github.io/cgroup2/docs/overview.html#case-study-the-fbtax2-project [akpm@linux-foundation.org: reflow block comments to fit in 80 cols] [chris@chrisdown.name: handle cgroup_disable=memory when getting memcg protection] Link: http://lkml.kernel.org/r/20190201045711.GA18302@chrisdown.name Link: http://lkml.kernel.org/r/20190124014455.GA6396@chrisdown.name Signed-off-by: Chris Down <chris@chrisdown.name> Acked-by: Johannes Weiner <hannes@cmpxchg.org> Reviewed-by: Roman Gushchin <guro@fb.com> Cc: Michal Hocko <mhocko@kernel.org> Cc: Tejun Heo <tj@kernel.org> Cc: Dennis Zhou <dennis@kernel.org> Cc: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2546 lines
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2546 lines
94 KiB
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================
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Control Group v2
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================
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:Date: October, 2015
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:Author: Tejun Heo <tj@kernel.org>
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This is the authoritative documentation on the design, interface and
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conventions of cgroup v2. It describes all userland-visible aspects
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of cgroup including core and specific controller behaviors. All
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future changes must be reflected in this document. Documentation for
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v1 is available under Documentation/admin-guide/cgroup-v1/.
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.. CONTENTS
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1. Introduction
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1-1. Terminology
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1-2. What is cgroup?
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2. Basic Operations
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2-1. Mounting
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2-2. Organizing Processes and Threads
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2-2-1. Processes
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2-2-2. Threads
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2-3. [Un]populated Notification
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2-4. Controlling Controllers
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2-4-1. Enabling and Disabling
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2-4-2. Top-down Constraint
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2-4-3. No Internal Process Constraint
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2-5. Delegation
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2-5-1. Model of Delegation
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2-5-2. Delegation Containment
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2-6. Guidelines
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2-6-1. Organize Once and Control
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2-6-2. Avoid Name Collisions
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3. Resource Distribution Models
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3-1. Weights
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3-2. Limits
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3-3. Protections
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3-4. Allocations
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4. Interface Files
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4-1. Format
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4-2. Conventions
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4-3. Core Interface Files
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5. Controllers
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5-1. CPU
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5-1-1. CPU Interface Files
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5-2. Memory
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5-2-1. Memory Interface Files
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5-2-2. Usage Guidelines
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5-2-3. Memory Ownership
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5-3. IO
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5-3-1. IO Interface Files
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5-3-2. Writeback
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5-3-3. IO Latency
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5-3-3-1. How IO Latency Throttling Works
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5-3-3-2. IO Latency Interface Files
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5-4. PID
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5-4-1. PID Interface Files
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5-5. Cpuset
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5.5-1. Cpuset Interface Files
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5-6. Device
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5-7. RDMA
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5-7-1. RDMA Interface Files
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5-8. Misc
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5-8-1. perf_event
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5-N. Non-normative information
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5-N-1. CPU controller root cgroup process behaviour
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5-N-2. IO controller root cgroup process behaviour
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6. Namespace
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6-1. Basics
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6-2. The Root and Views
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6-3. Migration and setns(2)
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6-4. Interaction with Other Namespaces
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P. Information on Kernel Programming
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P-1. Filesystem Support for Writeback
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D. Deprecated v1 Core Features
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R. Issues with v1 and Rationales for v2
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R-1. Multiple Hierarchies
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R-2. Thread Granularity
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R-3. Competition Between Inner Nodes and Threads
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R-4. Other Interface Issues
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R-5. Controller Issues and Remedies
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R-5-1. Memory
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Introduction
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============
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Terminology
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-----------
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"cgroup" stands for "control group" and is never capitalized. The
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singular form is used to designate the whole feature and also as a
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qualifier as in "cgroup controllers". When explicitly referring to
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multiple individual control groups, the plural form "cgroups" is used.
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What is cgroup?
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---------------
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cgroup is a mechanism to organize processes hierarchically and
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distribute system resources along the hierarchy in a controlled and
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configurable manner.
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cgroup is largely composed of two parts - the core and controllers.
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cgroup core is primarily responsible for hierarchically organizing
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processes. A cgroup controller is usually responsible for
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distributing a specific type of system resource along the hierarchy
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although there are utility controllers which serve purposes other than
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resource distribution.
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cgroups form a tree structure and every process in the system belongs
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to one and only one cgroup. All threads of a process belong to the
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same cgroup. On creation, all processes are put in the cgroup that
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the parent process belongs to at the time. A process can be migrated
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to another cgroup. Migration of a process doesn't affect already
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existing descendant processes.
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Following certain structural constraints, controllers may be enabled or
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disabled selectively on a cgroup. All controller behaviors are
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hierarchical - if a controller is enabled on a cgroup, it affects all
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processes which belong to the cgroups consisting the inclusive
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sub-hierarchy of the cgroup. When a controller is enabled on a nested
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cgroup, it always restricts the resource distribution further. The
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restrictions set closer to the root in the hierarchy can not be
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overridden from further away.
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Basic Operations
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================
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Mounting
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--------
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Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2
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hierarchy can be mounted with the following mount command::
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# mount -t cgroup2 none $MOUNT_POINT
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cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All
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controllers which support v2 and are not bound to a v1 hierarchy are
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automatically bound to the v2 hierarchy and show up at the root.
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Controllers which are not in active use in the v2 hierarchy can be
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bound to other hierarchies. This allows mixing v2 hierarchy with the
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legacy v1 multiple hierarchies in a fully backward compatible way.
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A controller can be moved across hierarchies only after the controller
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is no longer referenced in its current hierarchy. Because per-cgroup
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controller states are destroyed asynchronously and controllers may
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have lingering references, a controller may not show up immediately on
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the v2 hierarchy after the final umount of the previous hierarchy.
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Similarly, a controller should be fully disabled to be moved out of
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the unified hierarchy and it may take some time for the disabled
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controller to become available for other hierarchies; furthermore, due
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to inter-controller dependencies, other controllers may need to be
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disabled too.
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While useful for development and manual configurations, moving
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controllers dynamically between the v2 and other hierarchies is
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strongly discouraged for production use. It is recommended to decide
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the hierarchies and controller associations before starting using the
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controllers after system boot.
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During transition to v2, system management software might still
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automount the v1 cgroup filesystem and so hijack all controllers
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during boot, before manual intervention is possible. To make testing
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and experimenting easier, the kernel parameter cgroup_no_v1= allows
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disabling controllers in v1 and make them always available in v2.
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cgroup v2 currently supports the following mount options.
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nsdelegate
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Consider cgroup namespaces as delegation boundaries. This
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option is system wide and can only be set on mount or modified
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through remount from the init namespace. The mount option is
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ignored on non-init namespace mounts. Please refer to the
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Delegation section for details.
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memory_localevents
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Only populate memory.events with data for the current cgroup,
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and not any subtrees. This is legacy behaviour, the default
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behaviour without this option is to include subtree counts.
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This option is system wide and can only be set on mount or
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modified through remount from the init namespace. The mount
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option is ignored on non-init namespace mounts.
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Organizing Processes and Threads
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--------------------------------
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Processes
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~~~~~~~~~
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Initially, only the root cgroup exists to which all processes belong.
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A child cgroup can be created by creating a sub-directory::
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# mkdir $CGROUP_NAME
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A given cgroup may have multiple child cgroups forming a tree
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structure. Each cgroup has a read-writable interface file
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"cgroup.procs". When read, it lists the PIDs of all processes which
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belong to the cgroup one-per-line. The PIDs are not ordered and the
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same PID may show up more than once if the process got moved to
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another cgroup and then back or the PID got recycled while reading.
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A process can be migrated into a cgroup by writing its PID to the
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target cgroup's "cgroup.procs" file. Only one process can be migrated
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on a single write(2) call. If a process is composed of multiple
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threads, writing the PID of any thread migrates all threads of the
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process.
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When a process forks a child process, the new process is born into the
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cgroup that the forking process belongs to at the time of the
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operation. After exit, a process stays associated with the cgroup
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that it belonged to at the time of exit until it's reaped; however, a
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zombie process does not appear in "cgroup.procs" and thus can't be
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moved to another cgroup.
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A cgroup which doesn't have any children or live processes can be
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destroyed by removing the directory. Note that a cgroup which doesn't
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have any children and is associated only with zombie processes is
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considered empty and can be removed::
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# rmdir $CGROUP_NAME
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"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy
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cgroup is in use in the system, this file may contain multiple lines,
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one for each hierarchy. The entry for cgroup v2 is always in the
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format "0::$PATH"::
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# cat /proc/842/cgroup
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...
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0::/test-cgroup/test-cgroup-nested
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If the process becomes a zombie and the cgroup it was associated with
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is removed subsequently, " (deleted)" is appended to the path::
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# cat /proc/842/cgroup
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...
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0::/test-cgroup/test-cgroup-nested (deleted)
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Threads
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~~~~~~~
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cgroup v2 supports thread granularity for a subset of controllers to
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support use cases requiring hierarchical resource distribution across
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the threads of a group of processes. By default, all threads of a
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process belong to the same cgroup, which also serves as the resource
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domain to host resource consumptions which are not specific to a
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process or thread. The thread mode allows threads to be spread across
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a subtree while still maintaining the common resource domain for them.
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Controllers which support thread mode are called threaded controllers.
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The ones which don't are called domain controllers.
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Marking a cgroup threaded makes it join the resource domain of its
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parent as a threaded cgroup. The parent may be another threaded
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cgroup whose resource domain is further up in the hierarchy. The root
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of a threaded subtree, that is, the nearest ancestor which is not
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threaded, is called threaded domain or thread root interchangeably and
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serves as the resource domain for the entire subtree.
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Inside a threaded subtree, threads of a process can be put in
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different cgroups and are not subject to the no internal process
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constraint - threaded controllers can be enabled on non-leaf cgroups
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whether they have threads in them or not.
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As the threaded domain cgroup hosts all the domain resource
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consumptions of the subtree, it is considered to have internal
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resource consumptions whether there are processes in it or not and
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can't have populated child cgroups which aren't threaded. Because the
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root cgroup is not subject to no internal process constraint, it can
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serve both as a threaded domain and a parent to domain cgroups.
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The current operation mode or type of the cgroup is shown in the
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"cgroup.type" file which indicates whether the cgroup is a normal
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domain, a domain which is serving as the domain of a threaded subtree,
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or a threaded cgroup.
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On creation, a cgroup is always a domain cgroup and can be made
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threaded by writing "threaded" to the "cgroup.type" file. The
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operation is single direction::
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# echo threaded > cgroup.type
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Once threaded, the cgroup can't be made a domain again. To enable the
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thread mode, the following conditions must be met.
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- As the cgroup will join the parent's resource domain. The parent
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must either be a valid (threaded) domain or a threaded cgroup.
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- When the parent is an unthreaded domain, it must not have any domain
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controllers enabled or populated domain children. The root is
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exempt from this requirement.
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Topology-wise, a cgroup can be in an invalid state. Please consider
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the following topology::
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A (threaded domain) - B (threaded) - C (domain, just created)
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C is created as a domain but isn't connected to a parent which can
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host child domains. C can't be used until it is turned into a
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threaded cgroup. "cgroup.type" file will report "domain (invalid)" in
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these cases. Operations which fail due to invalid topology use
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EOPNOTSUPP as the errno.
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A domain cgroup is turned into a threaded domain when one of its child
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cgroup becomes threaded or threaded controllers are enabled in the
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"cgroup.subtree_control" file while there are processes in the cgroup.
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A threaded domain reverts to a normal domain when the conditions
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clear.
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When read, "cgroup.threads" contains the list of the thread IDs of all
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threads in the cgroup. Except that the operations are per-thread
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instead of per-process, "cgroup.threads" has the same format and
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behaves the same way as "cgroup.procs". While "cgroup.threads" can be
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written to in any cgroup, as it can only move threads inside the same
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threaded domain, its operations are confined inside each threaded
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subtree.
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The threaded domain cgroup serves as the resource domain for the whole
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subtree, and, while the threads can be scattered across the subtree,
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all the processes are considered to be in the threaded domain cgroup.
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"cgroup.procs" in a threaded domain cgroup contains the PIDs of all
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processes in the subtree and is not readable in the subtree proper.
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However, "cgroup.procs" can be written to from anywhere in the subtree
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to migrate all threads of the matching process to the cgroup.
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Only threaded controllers can be enabled in a threaded subtree. When
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a threaded controller is enabled inside a threaded subtree, it only
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accounts for and controls resource consumptions associated with the
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threads in the cgroup and its descendants. All consumptions which
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aren't tied to a specific thread belong to the threaded domain cgroup.
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Because a threaded subtree is exempt from no internal process
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constraint, a threaded controller must be able to handle competition
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between threads in a non-leaf cgroup and its child cgroups. Each
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threaded controller defines how such competitions are handled.
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[Un]populated Notification
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--------------------------
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Each non-root cgroup has a "cgroup.events" file which contains
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"populated" field indicating whether the cgroup's sub-hierarchy has
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live processes in it. Its value is 0 if there is no live process in
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the cgroup and its descendants; otherwise, 1. poll and [id]notify
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events are triggered when the value changes. This can be used, for
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example, to start a clean-up operation after all processes of a given
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sub-hierarchy have exited. The populated state updates and
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notifications are recursive. Consider the following sub-hierarchy
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where the numbers in the parentheses represent the numbers of processes
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in each cgroup::
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A(4) - B(0) - C(1)
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\ D(0)
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A, B and C's "populated" fields would be 1 while D's 0. After the one
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process in C exits, B and C's "populated" fields would flip to "0" and
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file modified events will be generated on the "cgroup.events" files of
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both cgroups.
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Controlling Controllers
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-----------------------
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Enabling and Disabling
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~~~~~~~~~~~~~~~~~~~~~~
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Each cgroup has a "cgroup.controllers" file which lists all
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controllers available for the cgroup to enable::
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# cat cgroup.controllers
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cpu io memory
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No controller is enabled by default. Controllers can be enabled and
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disabled by writing to the "cgroup.subtree_control" file::
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# echo "+cpu +memory -io" > cgroup.subtree_control
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Only controllers which are listed in "cgroup.controllers" can be
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enabled. When multiple operations are specified as above, either they
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all succeed or fail. If multiple operations on the same controller
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are specified, the last one is effective.
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Enabling a controller in a cgroup indicates that the distribution of
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the target resource across its immediate children will be controlled.
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Consider the following sub-hierarchy. The enabled controllers are
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listed in parentheses::
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A(cpu,memory) - B(memory) - C()
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\ D()
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As A has "cpu" and "memory" enabled, A will control the distribution
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of CPU cycles and memory to its children, in this case, B. As B has
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"memory" enabled but not "CPU", C and D will compete freely on CPU
|
|
cycles but their division of memory available to B will be controlled.
|
|
|
|
As a controller regulates the distribution of the target resource to
|
|
the cgroup's children, enabling it creates the controller's interface
|
|
files in the child cgroups. In the above example, enabling "cpu" on B
|
|
would create the "cpu." prefixed controller interface files in C and
|
|
D. Likewise, disabling "memory" from B would remove the "memory."
|
|
prefixed controller interface files from C and D. This means that the
|
|
controller interface files - anything which doesn't start with
|
|
"cgroup." are owned by the parent rather than the cgroup itself.
|
|
|
|
|
|
Top-down Constraint
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
Resources are distributed top-down and a cgroup can further distribute
|
|
a resource only if the resource has been distributed to it from the
|
|
parent. This means that all non-root "cgroup.subtree_control" files
|
|
can only contain controllers which are enabled in the parent's
|
|
"cgroup.subtree_control" file. A controller can be enabled only if
|
|
the parent has the controller enabled and a controller can't be
|
|
disabled if one or more children have it enabled.
|
|
|
|
|
|
No Internal Process Constraint
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Non-root cgroups can distribute domain resources to their children
|
|
only when they don't have any processes of their own. In other words,
|
|
only domain cgroups which don't contain any processes can have domain
|
|
controllers enabled in their "cgroup.subtree_control" files.
|
|
|
|
This guarantees that, when a domain controller is looking at the part
|
|
of the hierarchy which has it enabled, processes are always only on
|
|
the leaves. This rules out situations where child cgroups compete
|
|
against internal processes of the parent.
|
|
|
|
The root cgroup is exempt from this restriction. Root contains
|
|
processes and anonymous resource consumption which can't be associated
|
|
with any other cgroups and requires special treatment from most
|
|
controllers. How resource consumption in the root cgroup is governed
|
|
is up to each controller (for more information on this topic please
|
|
refer to the Non-normative information section in the Controllers
|
|
chapter).
|
|
|
|
Note that the restriction doesn't get in the way if there is no
|
|
enabled controller in the cgroup's "cgroup.subtree_control". This is
|
|
important as otherwise it wouldn't be possible to create children of a
|
|
populated cgroup. To control resource distribution of a cgroup, the
|
|
cgroup must create children and transfer all its processes to the
|
|
children before enabling controllers in its "cgroup.subtree_control"
|
|
file.
|
|
|
|
|
|
Delegation
|
|
----------
|
|
|
|
Model of Delegation
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
A cgroup can be delegated in two ways. First, to a less privileged
|
|
user by granting write access of the directory and its "cgroup.procs",
|
|
"cgroup.threads" and "cgroup.subtree_control" files to the user.
|
|
Second, if the "nsdelegate" mount option is set, automatically to a
|
|
cgroup namespace on namespace creation.
|
|
|
|
Because the resource control interface files in a given directory
|
|
control the distribution of the parent's resources, the delegatee
|
|
shouldn't be allowed to write to them. For the first method, this is
|
|
achieved by not granting access to these files. For the second, the
|
|
kernel rejects writes to all files other than "cgroup.procs" and
|
|
"cgroup.subtree_control" on a namespace root from inside the
|
|
namespace.
|
|
|
|
The end results are equivalent for both delegation types. Once
|
|
delegated, the user can build sub-hierarchy under the directory,
|
|
organize processes inside it as it sees fit and further distribute the
|
|
resources it received from the parent. The limits and other settings
|
|
of all resource controllers are hierarchical and regardless of what
|
|
happens in the delegated sub-hierarchy, nothing can escape the
|
|
resource restrictions imposed by the parent.
|
|
|
|
Currently, cgroup doesn't impose any restrictions on the number of
|
|
cgroups in or nesting depth of a delegated sub-hierarchy; however,
|
|
this may be limited explicitly in the future.
|
|
|
|
|
|
Delegation Containment
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
A delegated sub-hierarchy is contained in the sense that processes
|
|
can't be moved into or out of the sub-hierarchy by the delegatee.
|
|
|
|
For delegations to a less privileged user, this is achieved by
|
|
requiring the following conditions for a process with a non-root euid
|
|
to migrate a target process into a cgroup by writing its PID to the
|
|
"cgroup.procs" file.
|
|
|
|
- The writer must have write access to the "cgroup.procs" file.
|
|
|
|
- The writer must have write access to the "cgroup.procs" file of the
|
|
common ancestor of the source and destination cgroups.
|
|
|
|
The above two constraints ensure that while a delegatee may migrate
|
|
processes around freely in the delegated sub-hierarchy it can't pull
|
|
in from or push out to outside the sub-hierarchy.
|
|
|
|
For an example, let's assume cgroups C0 and C1 have been delegated to
|
|
user U0 who created C00, C01 under C0 and C10 under C1 as follows and
|
|
all processes under C0 and C1 belong to U0::
|
|
|
|
~~~~~~~~~~~~~ - C0 - C00
|
|
~ cgroup ~ \ C01
|
|
~ hierarchy ~
|
|
~~~~~~~~~~~~~ - C1 - C10
|
|
|
|
Let's also say U0 wants to write the PID of a process which is
|
|
currently in C10 into "C00/cgroup.procs". U0 has write access to the
|
|
file; however, the common ancestor of the source cgroup C10 and the
|
|
destination cgroup C00 is above the points of delegation and U0 would
|
|
not have write access to its "cgroup.procs" files and thus the write
|
|
will be denied with -EACCES.
|
|
|
|
For delegations to namespaces, containment is achieved by requiring
|
|
that both the source and destination cgroups are reachable from the
|
|
namespace of the process which is attempting the migration. If either
|
|
is not reachable, the migration is rejected with -ENOENT.
|
|
|
|
|
|
Guidelines
|
|
----------
|
|
|
|
Organize Once and Control
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Migrating a process across cgroups is a relatively expensive operation
|
|
and stateful resources such as memory are not moved together with the
|
|
process. This is an explicit design decision as there often exist
|
|
inherent trade-offs between migration and various hot paths in terms
|
|
of synchronization cost.
|
|
|
|
As such, migrating processes across cgroups frequently as a means to
|
|
apply different resource restrictions is discouraged. A workload
|
|
should be assigned to a cgroup according to the system's logical and
|
|
resource structure once on start-up. Dynamic adjustments to resource
|
|
distribution can be made by changing controller configuration through
|
|
the interface files.
|
|
|
|
|
|
Avoid Name Collisions
|
|
~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Interface files for a cgroup and its children cgroups occupy the same
|
|
directory and it is possible to create children cgroups which collide
|
|
with interface files.
|
|
|
|
All cgroup core interface files are prefixed with "cgroup." and each
|
|
controller's interface files are prefixed with the controller name and
|
|
a dot. A controller's name is composed of lower case alphabets and
|
|
'_'s but never begins with an '_' so it can be used as the prefix
|
|
character for collision avoidance. Also, interface file names won't
|
|
start or end with terms which are often used in categorizing workloads
|
|
such as job, service, slice, unit or workload.
|
|
|
|
cgroup doesn't do anything to prevent name collisions and it's the
|
|
user's responsibility to avoid them.
|
|
|
|
|
|
Resource Distribution Models
|
|
============================
|
|
|
|
cgroup controllers implement several resource distribution schemes
|
|
depending on the resource type and expected use cases. This section
|
|
describes major schemes in use along with their expected behaviors.
|
|
|
|
|
|
Weights
|
|
-------
|
|
|
|
A parent's resource is distributed by adding up the weights of all
|
|
active children and giving each the fraction matching the ratio of its
|
|
weight against the sum. As only children which can make use of the
|
|
resource at the moment participate in the distribution, this is
|
|
work-conserving. Due to the dynamic nature, this model is usually
|
|
used for stateless resources.
|
|
|
|
All weights are in the range [1, 10000] with the default at 100. This
|
|
allows symmetric multiplicative biases in both directions at fine
|
|
enough granularity while staying in the intuitive range.
|
|
|
|
As long as the weight is in range, all configuration combinations are
|
|
valid and there is no reason to reject configuration changes or
|
|
process migrations.
|
|
|
|
"cpu.weight" proportionally distributes CPU cycles to active children
|
|
and is an example of this type.
|
|
|
|
|
|
Limits
|
|
------
|
|
|
|
A child can only consume upto the configured amount of the resource.
|
|
Limits can be over-committed - the sum of the limits of children can
|
|
exceed the amount of resource available to the parent.
|
|
|
|
Limits are in the range [0, max] and defaults to "max", which is noop.
|
|
|
|
As limits can be over-committed, all configuration combinations are
|
|
valid and there is no reason to reject configuration changes or
|
|
process migrations.
|
|
|
|
"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
|
|
on an IO device and is an example of this type.
|
|
|
|
|
|
Protections
|
|
-----------
|
|
|
|
A cgroup is protected upto the configured amount of the resource
|
|
as long as the usages of all its ancestors are under their
|
|
protected levels. Protections can be hard guarantees or best effort
|
|
soft boundaries. Protections can also be over-committed in which case
|
|
only upto the amount available to the parent is protected among
|
|
children.
|
|
|
|
Protections are in the range [0, max] and defaults to 0, which is
|
|
noop.
|
|
|
|
As protections can be over-committed, all configuration combinations
|
|
are valid and there is no reason to reject configuration changes or
|
|
process migrations.
|
|
|
|
"memory.low" implements best-effort memory protection and is an
|
|
example of this type.
|
|
|
|
|
|
Allocations
|
|
-----------
|
|
|
|
A cgroup is exclusively allocated a certain amount of a finite
|
|
resource. Allocations can't be over-committed - the sum of the
|
|
allocations of children can not exceed the amount of resource
|
|
available to the parent.
|
|
|
|
Allocations are in the range [0, max] and defaults to 0, which is no
|
|
resource.
|
|
|
|
As allocations can't be over-committed, some configuration
|
|
combinations are invalid and should be rejected. Also, if the
|
|
resource is mandatory for execution of processes, process migrations
|
|
may be rejected.
|
|
|
|
"cpu.rt.max" hard-allocates realtime slices and is an example of this
|
|
type.
|
|
|
|
|
|
Interface Files
|
|
===============
|
|
|
|
Format
|
|
------
|
|
|
|
All interface files should be in one of the following formats whenever
|
|
possible::
|
|
|
|
New-line separated values
|
|
(when only one value can be written at once)
|
|
|
|
VAL0\n
|
|
VAL1\n
|
|
...
|
|
|
|
Space separated values
|
|
(when read-only or multiple values can be written at once)
|
|
|
|
VAL0 VAL1 ...\n
|
|
|
|
Flat keyed
|
|
|
|
KEY0 VAL0\n
|
|
KEY1 VAL1\n
|
|
...
|
|
|
|
Nested keyed
|
|
|
|
KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
|
|
KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
|
|
...
|
|
|
|
For a writable file, the format for writing should generally match
|
|
reading; however, controllers may allow omitting later fields or
|
|
implement restricted shortcuts for most common use cases.
|
|
|
|
For both flat and nested keyed files, only the values for a single key
|
|
can be written at a time. For nested keyed files, the sub key pairs
|
|
may be specified in any order and not all pairs have to be specified.
|
|
|
|
|
|
Conventions
|
|
-----------
|
|
|
|
- Settings for a single feature should be contained in a single file.
|
|
|
|
- The root cgroup should be exempt from resource control and thus
|
|
shouldn't have resource control interface files. Also,
|
|
informational files on the root cgroup which end up showing global
|
|
information available elsewhere shouldn't exist.
|
|
|
|
- The default time unit is microseconds. If a different unit is ever
|
|
used, an explicit unit suffix must be present.
|
|
|
|
- A parts-per quantity should use a percentage decimal with at least
|
|
two digit fractional part - e.g. 13.40.
|
|
|
|
- If a controller implements weight based resource distribution, its
|
|
interface file should be named "weight" and have the range [1,
|
|
10000] with 100 as the default. The values are chosen to allow
|
|
enough and symmetric bias in both directions while keeping it
|
|
intuitive (the default is 100%).
|
|
|
|
- If a controller implements an absolute resource guarantee and/or
|
|
limit, the interface files should be named "min" and "max"
|
|
respectively. If a controller implements best effort resource
|
|
guarantee and/or limit, the interface files should be named "low"
|
|
and "high" respectively.
|
|
|
|
In the above four control files, the special token "max" should be
|
|
used to represent upward infinity for both reading and writing.
|
|
|
|
- If a setting has a configurable default value and keyed specific
|
|
overrides, the default entry should be keyed with "default" and
|
|
appear as the first entry in the file.
|
|
|
|
The default value can be updated by writing either "default $VAL" or
|
|
"$VAL".
|
|
|
|
When writing to update a specific override, "default" can be used as
|
|
the value to indicate removal of the override. Override entries
|
|
with "default" as the value must not appear when read.
|
|
|
|
For example, a setting which is keyed by major:minor device numbers
|
|
with integer values may look like the following::
|
|
|
|
# cat cgroup-example-interface-file
|
|
default 150
|
|
8:0 300
|
|
|
|
The default value can be updated by::
|
|
|
|
# echo 125 > cgroup-example-interface-file
|
|
|
|
or::
|
|
|
|
# echo "default 125" > cgroup-example-interface-file
|
|
|
|
An override can be set by::
|
|
|
|
# echo "8:16 170" > cgroup-example-interface-file
|
|
|
|
and cleared by::
|
|
|
|
# echo "8:0 default" > cgroup-example-interface-file
|
|
# cat cgroup-example-interface-file
|
|
default 125
|
|
8:16 170
|
|
|
|
- For events which are not very high frequency, an interface file
|
|
"events" should be created which lists event key value pairs.
|
|
Whenever a notifiable event happens, file modified event should be
|
|
generated on the file.
|
|
|
|
|
|
Core Interface Files
|
|
--------------------
|
|
|
|
All cgroup core files are prefixed with "cgroup."
|
|
|
|
cgroup.type
|
|
|
|
A read-write single value file which exists on non-root
|
|
cgroups.
|
|
|
|
When read, it indicates the current type of the cgroup, which
|
|
can be one of the following values.
|
|
|
|
- "domain" : A normal valid domain cgroup.
|
|
|
|
- "domain threaded" : A threaded domain cgroup which is
|
|
serving as the root of a threaded subtree.
|
|
|
|
- "domain invalid" : A cgroup which is in an invalid state.
|
|
It can't be populated or have controllers enabled. It may
|
|
be allowed to become a threaded cgroup.
|
|
|
|
- "threaded" : A threaded cgroup which is a member of a
|
|
threaded subtree.
|
|
|
|
A cgroup can be turned into a threaded cgroup by writing
|
|
"threaded" to this file.
|
|
|
|
cgroup.procs
|
|
A read-write new-line separated values file which exists on
|
|
all cgroups.
|
|
|
|
When read, it lists the PIDs of all processes which belong to
|
|
the cgroup one-per-line. The PIDs are not ordered and the
|
|
same PID may show up more than once if the process got moved
|
|
to another cgroup and then back or the PID got recycled while
|
|
reading.
|
|
|
|
A PID can be written to migrate the process associated with
|
|
the PID to the cgroup. The writer should match all of the
|
|
following conditions.
|
|
|
|
- It must have write access to the "cgroup.procs" file.
|
|
|
|
- It must have write access to the "cgroup.procs" file of the
|
|
common ancestor of the source and destination cgroups.
|
|
|
|
When delegating a sub-hierarchy, write access to this file
|
|
should be granted along with the containing directory.
|
|
|
|
In a threaded cgroup, reading this file fails with EOPNOTSUPP
|
|
as all the processes belong to the thread root. Writing is
|
|
supported and moves every thread of the process to the cgroup.
|
|
|
|
cgroup.threads
|
|
A read-write new-line separated values file which exists on
|
|
all cgroups.
|
|
|
|
When read, it lists the TIDs of all threads which belong to
|
|
the cgroup one-per-line. The TIDs are not ordered and the
|
|
same TID may show up more than once if the thread got moved to
|
|
another cgroup and then back or the TID got recycled while
|
|
reading.
|
|
|
|
A TID can be written to migrate the thread associated with the
|
|
TID to the cgroup. The writer should match all of the
|
|
following conditions.
|
|
|
|
- It must have write access to the "cgroup.threads" file.
|
|
|
|
- The cgroup that the thread is currently in must be in the
|
|
same resource domain as the destination cgroup.
|
|
|
|
- It must have write access to the "cgroup.procs" file of the
|
|
common ancestor of the source and destination cgroups.
|
|
|
|
When delegating a sub-hierarchy, write access to this file
|
|
should be granted along with the containing directory.
|
|
|
|
cgroup.controllers
|
|
A read-only space separated values file which exists on all
|
|
cgroups.
|
|
|
|
It shows space separated list of all controllers available to
|
|
the cgroup. The controllers are not ordered.
|
|
|
|
cgroup.subtree_control
|
|
A read-write space separated values file which exists on all
|
|
cgroups. Starts out empty.
|
|
|
|
When read, it shows space separated list of the controllers
|
|
which are enabled to control resource distribution from the
|
|
cgroup to its children.
|
|
|
|
Space separated list of controllers prefixed with '+' or '-'
|
|
can be written to enable or disable controllers. A controller
|
|
name prefixed with '+' enables the controller and '-'
|
|
disables. If a controller appears more than once on the list,
|
|
the last one is effective. When multiple enable and disable
|
|
operations are specified, either all succeed or all fail.
|
|
|
|
cgroup.events
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
The following entries are defined. Unless specified
|
|
otherwise, a value change in this file generates a file
|
|
modified event.
|
|
|
|
populated
|
|
1 if the cgroup or its descendants contains any live
|
|
processes; otherwise, 0.
|
|
frozen
|
|
1 if the cgroup is frozen; otherwise, 0.
|
|
|
|
cgroup.max.descendants
|
|
A read-write single value files. The default is "max".
|
|
|
|
Maximum allowed number of descent cgroups.
|
|
If the actual number of descendants is equal or larger,
|
|
an attempt to create a new cgroup in the hierarchy will fail.
|
|
|
|
cgroup.max.depth
|
|
A read-write single value files. The default is "max".
|
|
|
|
Maximum allowed descent depth below the current cgroup.
|
|
If the actual descent depth is equal or larger,
|
|
an attempt to create a new child cgroup will fail.
|
|
|
|
cgroup.stat
|
|
A read-only flat-keyed file with the following entries:
|
|
|
|
nr_descendants
|
|
Total number of visible descendant cgroups.
|
|
|
|
nr_dying_descendants
|
|
Total number of dying descendant cgroups. A cgroup becomes
|
|
dying after being deleted by a user. The cgroup will remain
|
|
in dying state for some time undefined time (which can depend
|
|
on system load) before being completely destroyed.
|
|
|
|
A process can't enter a dying cgroup under any circumstances,
|
|
a dying cgroup can't revive.
|
|
|
|
A dying cgroup can consume system resources not exceeding
|
|
limits, which were active at the moment of cgroup deletion.
|
|
|
|
cgroup.freeze
|
|
A read-write single value file which exists on non-root cgroups.
|
|
Allowed values are "0" and "1". The default is "0".
|
|
|
|
Writing "1" to the file causes freezing of the cgroup and all
|
|
descendant cgroups. This means that all belonging processes will
|
|
be stopped and will not run until the cgroup will be explicitly
|
|
unfrozen. Freezing of the cgroup may take some time; when this action
|
|
is completed, the "frozen" value in the cgroup.events control file
|
|
will be updated to "1" and the corresponding notification will be
|
|
issued.
|
|
|
|
A cgroup can be frozen either by its own settings, or by settings
|
|
of any ancestor cgroups. If any of ancestor cgroups is frozen, the
|
|
cgroup will remain frozen.
|
|
|
|
Processes in the frozen cgroup can be killed by a fatal signal.
|
|
They also can enter and leave a frozen cgroup: either by an explicit
|
|
move by a user, or if freezing of the cgroup races with fork().
|
|
If a process is moved to a frozen cgroup, it stops. If a process is
|
|
moved out of a frozen cgroup, it becomes running.
|
|
|
|
Frozen status of a cgroup doesn't affect any cgroup tree operations:
|
|
it's possible to delete a frozen (and empty) cgroup, as well as
|
|
create new sub-cgroups.
|
|
|
|
Controllers
|
|
===========
|
|
|
|
CPU
|
|
---
|
|
|
|
The "cpu" controllers regulates distribution of CPU cycles. This
|
|
controller implements weight and absolute bandwidth limit models for
|
|
normal scheduling policy and absolute bandwidth allocation model for
|
|
realtime scheduling policy.
|
|
|
|
In all the above models, cycles distribution is defined only on a temporal
|
|
base and it does not account for the frequency at which tasks are executed.
|
|
The (optional) utilization clamping support allows to hint the schedutil
|
|
cpufreq governor about the minimum desired frequency which should always be
|
|
provided by a CPU, as well as the maximum desired frequency, which should not
|
|
be exceeded by a CPU.
|
|
|
|
WARNING: cgroup2 doesn't yet support control of realtime processes and
|
|
the cpu controller can only be enabled when all RT processes are in
|
|
the root cgroup. Be aware that system management software may already
|
|
have placed RT processes into nonroot cgroups during the system boot
|
|
process, and these processes may need to be moved to the root cgroup
|
|
before the cpu controller can be enabled.
|
|
|
|
|
|
CPU Interface Files
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
All time durations are in microseconds.
|
|
|
|
cpu.stat
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
This file exists whether the controller is enabled or not.
|
|
|
|
It always reports the following three stats:
|
|
|
|
- usage_usec
|
|
- user_usec
|
|
- system_usec
|
|
|
|
and the following three when the controller is enabled:
|
|
|
|
- nr_periods
|
|
- nr_throttled
|
|
- throttled_usec
|
|
|
|
cpu.weight
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "100".
|
|
|
|
The weight in the range [1, 10000].
|
|
|
|
cpu.weight.nice
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "0".
|
|
|
|
The nice value is in the range [-20, 19].
|
|
|
|
This interface file is an alternative interface for
|
|
"cpu.weight" and allows reading and setting weight using the
|
|
same values used by nice(2). Because the range is smaller and
|
|
granularity is coarser for the nice values, the read value is
|
|
the closest approximation of the current weight.
|
|
|
|
cpu.max
|
|
A read-write two value file which exists on non-root cgroups.
|
|
The default is "max 100000".
|
|
|
|
The maximum bandwidth limit. It's in the following format::
|
|
|
|
$MAX $PERIOD
|
|
|
|
which indicates that the group may consume upto $MAX in each
|
|
$PERIOD duration. "max" for $MAX indicates no limit. If only
|
|
one number is written, $MAX is updated.
|
|
|
|
cpu.pressure
|
|
A read-only nested-key file which exists on non-root cgroups.
|
|
|
|
Shows pressure stall information for CPU. See
|
|
Documentation/accounting/psi.rst for details.
|
|
|
|
cpu.uclamp.min
|
|
A read-write single value file which exists on non-root cgroups.
|
|
The default is "0", i.e. no utilization boosting.
|
|
|
|
The requested minimum utilization (protection) as a percentage
|
|
rational number, e.g. 12.34 for 12.34%.
|
|
|
|
This interface allows reading and setting minimum utilization clamp
|
|
values similar to the sched_setattr(2). This minimum utilization
|
|
value is used to clamp the task specific minimum utilization clamp.
|
|
|
|
The requested minimum utilization (protection) is always capped by
|
|
the current value for the maximum utilization (limit), i.e.
|
|
`cpu.uclamp.max`.
|
|
|
|
cpu.uclamp.max
|
|
A read-write single value file which exists on non-root cgroups.
|
|
The default is "max". i.e. no utilization capping
|
|
|
|
The requested maximum utilization (limit) as a percentage rational
|
|
number, e.g. 98.76 for 98.76%.
|
|
|
|
This interface allows reading and setting maximum utilization clamp
|
|
values similar to the sched_setattr(2). This maximum utilization
|
|
value is used to clamp the task specific maximum utilization clamp.
|
|
|
|
|
|
|
|
Memory
|
|
------
|
|
|
|
The "memory" controller regulates distribution of memory. Memory is
|
|
stateful and implements both limit and protection models. Due to the
|
|
intertwining between memory usage and reclaim pressure and the
|
|
stateful nature of memory, the distribution model is relatively
|
|
complex.
|
|
|
|
While not completely water-tight, all major memory usages by a given
|
|
cgroup are tracked so that the total memory consumption can be
|
|
accounted and controlled to a reasonable extent. Currently, the
|
|
following types of memory usages are tracked.
|
|
|
|
- Userland memory - page cache and anonymous memory.
|
|
|
|
- Kernel data structures such as dentries and inodes.
|
|
|
|
- TCP socket buffers.
|
|
|
|
The above list may expand in the future for better coverage.
|
|
|
|
|
|
Memory Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
All memory amounts are in bytes. If a value which is not aligned to
|
|
PAGE_SIZE is written, the value may be rounded up to the closest
|
|
PAGE_SIZE multiple when read back.
|
|
|
|
memory.current
|
|
A read-only single value file which exists on non-root
|
|
cgroups.
|
|
|
|
The total amount of memory currently being used by the cgroup
|
|
and its descendants.
|
|
|
|
memory.min
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "0".
|
|
|
|
Hard memory protection. If the memory usage of a cgroup
|
|
is within its effective min boundary, the cgroup's memory
|
|
won't be reclaimed under any conditions. If there is no
|
|
unprotected reclaimable memory available, OOM killer
|
|
is invoked. Above the effective min boundary (or
|
|
effective low boundary if it is higher), pages are reclaimed
|
|
proportionally to the overage, reducing reclaim pressure for
|
|
smaller overages.
|
|
|
|
Effective min boundary is limited by memory.min values of
|
|
all ancestor cgroups. If there is memory.min overcommitment
|
|
(child cgroup or cgroups are requiring more protected memory
|
|
than parent will allow), then each child cgroup will get
|
|
the part of parent's protection proportional to its
|
|
actual memory usage below memory.min.
|
|
|
|
Putting more memory than generally available under this
|
|
protection is discouraged and may lead to constant OOMs.
|
|
|
|
If a memory cgroup is not populated with processes,
|
|
its memory.min is ignored.
|
|
|
|
memory.low
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "0".
|
|
|
|
Best-effort memory protection. If the memory usage of a
|
|
cgroup is within its effective low boundary, the cgroup's
|
|
memory won't be reclaimed unless memory can be reclaimed
|
|
from unprotected cgroups. Above the effective low boundary (or
|
|
effective min boundary if it is higher), pages are reclaimed
|
|
proportionally to the overage, reducing reclaim pressure for
|
|
smaller overages.
|
|
|
|
Effective low boundary is limited by memory.low values of
|
|
all ancestor cgroups. If there is memory.low overcommitment
|
|
(child cgroup or cgroups are requiring more protected memory
|
|
than parent will allow), then each child cgroup will get
|
|
the part of parent's protection proportional to its
|
|
actual memory usage below memory.low.
|
|
|
|
Putting more memory than generally available under this
|
|
protection is discouraged.
|
|
|
|
memory.high
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Memory usage throttle limit. This is the main mechanism to
|
|
control memory usage of a cgroup. If a cgroup's usage goes
|
|
over the high boundary, the processes of the cgroup are
|
|
throttled and put under heavy reclaim pressure.
|
|
|
|
Going over the high limit never invokes the OOM killer and
|
|
under extreme conditions the limit may be breached.
|
|
|
|
memory.max
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Memory usage hard limit. This is the final protection
|
|
mechanism. If a cgroup's memory usage reaches this limit and
|
|
can't be reduced, the OOM killer is invoked in the cgroup.
|
|
Under certain circumstances, the usage may go over the limit
|
|
temporarily.
|
|
|
|
This is the ultimate protection mechanism. As long as the
|
|
high limit is used and monitored properly, this limit's
|
|
utility is limited to providing the final safety net.
|
|
|
|
memory.oom.group
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default value is "0".
|
|
|
|
Determines whether the cgroup should be treated as
|
|
an indivisible workload by the OOM killer. If set,
|
|
all tasks belonging to the cgroup or to its descendants
|
|
(if the memory cgroup is not a leaf cgroup) are killed
|
|
together or not at all. This can be used to avoid
|
|
partial kills to guarantee workload integrity.
|
|
|
|
Tasks with the OOM protection (oom_score_adj set to -1000)
|
|
are treated as an exception and are never killed.
|
|
|
|
If the OOM killer is invoked in a cgroup, it's not going
|
|
to kill any tasks outside of this cgroup, regardless
|
|
memory.oom.group values of ancestor cgroups.
|
|
|
|
memory.events
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
The following entries are defined. Unless specified
|
|
otherwise, a value change in this file generates a file
|
|
modified event.
|
|
|
|
Note that all fields in this file are hierarchical and the
|
|
file modified event can be generated due to an event down the
|
|
hierarchy. For for the local events at the cgroup level see
|
|
memory.events.local.
|
|
|
|
low
|
|
The number of times the cgroup is reclaimed due to
|
|
high memory pressure even though its usage is under
|
|
the low boundary. This usually indicates that the low
|
|
boundary is over-committed.
|
|
|
|
high
|
|
The number of times processes of the cgroup are
|
|
throttled and routed to perform direct memory reclaim
|
|
because the high memory boundary was exceeded. For a
|
|
cgroup whose memory usage is capped by the high limit
|
|
rather than global memory pressure, this event's
|
|
occurrences are expected.
|
|
|
|
max
|
|
The number of times the cgroup's memory usage was
|
|
about to go over the max boundary. If direct reclaim
|
|
fails to bring it down, the cgroup goes to OOM state.
|
|
|
|
oom
|
|
The number of time the cgroup's memory usage was
|
|
reached the limit and allocation was about to fail.
|
|
|
|
Depending on context result could be invocation of OOM
|
|
killer and retrying allocation or failing allocation.
|
|
|
|
Failed allocation in its turn could be returned into
|
|
userspace as -ENOMEM or silently ignored in cases like
|
|
disk readahead. For now OOM in memory cgroup kills
|
|
tasks iff shortage has happened inside page fault.
|
|
|
|
This event is not raised if the OOM killer is not
|
|
considered as an option, e.g. for failed high-order
|
|
allocations.
|
|
|
|
oom_kill
|
|
The number of processes belonging to this cgroup
|
|
killed by any kind of OOM killer.
|
|
|
|
memory.events.local
|
|
Similar to memory.events but the fields in the file are local
|
|
to the cgroup i.e. not hierarchical. The file modified event
|
|
generated on this file reflects only the local events.
|
|
|
|
memory.stat
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
|
|
This breaks down the cgroup's memory footprint into different
|
|
types of memory, type-specific details, and other information
|
|
on the state and past events of the memory management system.
|
|
|
|
All memory amounts are in bytes.
|
|
|
|
The entries are ordered to be human readable, and new entries
|
|
can show up in the middle. Don't rely on items remaining in a
|
|
fixed position; use the keys to look up specific values!
|
|
|
|
anon
|
|
Amount of memory used in anonymous mappings such as
|
|
brk(), sbrk(), and mmap(MAP_ANONYMOUS)
|
|
|
|
file
|
|
Amount of memory used to cache filesystem data,
|
|
including tmpfs and shared memory.
|
|
|
|
kernel_stack
|
|
Amount of memory allocated to kernel stacks.
|
|
|
|
slab
|
|
Amount of memory used for storing in-kernel data
|
|
structures.
|
|
|
|
sock
|
|
Amount of memory used in network transmission buffers
|
|
|
|
shmem
|
|
Amount of cached filesystem data that is swap-backed,
|
|
such as tmpfs, shm segments, shared anonymous mmap()s
|
|
|
|
file_mapped
|
|
Amount of cached filesystem data mapped with mmap()
|
|
|
|
file_dirty
|
|
Amount of cached filesystem data that was modified but
|
|
not yet written back to disk
|
|
|
|
file_writeback
|
|
Amount of cached filesystem data that was modified and
|
|
is currently being written back to disk
|
|
|
|
anon_thp
|
|
Amount of memory used in anonymous mappings backed by
|
|
transparent hugepages
|
|
|
|
inactive_anon, active_anon, inactive_file, active_file, unevictable
|
|
Amount of memory, swap-backed and filesystem-backed,
|
|
on the internal memory management lists used by the
|
|
page reclaim algorithm
|
|
|
|
slab_reclaimable
|
|
Part of "slab" that might be reclaimed, such as
|
|
dentries and inodes.
|
|
|
|
slab_unreclaimable
|
|
Part of "slab" that cannot be reclaimed on memory
|
|
pressure.
|
|
|
|
pgfault
|
|
Total number of page faults incurred
|
|
|
|
pgmajfault
|
|
Number of major page faults incurred
|
|
|
|
workingset_refault
|
|
|
|
Number of refaults of previously evicted pages
|
|
|
|
workingset_activate
|
|
|
|
Number of refaulted pages that were immediately activated
|
|
|
|
workingset_nodereclaim
|
|
|
|
Number of times a shadow node has been reclaimed
|
|
|
|
pgrefill
|
|
|
|
Amount of scanned pages (in an active LRU list)
|
|
|
|
pgscan
|
|
|
|
Amount of scanned pages (in an inactive LRU list)
|
|
|
|
pgsteal
|
|
|
|
Amount of reclaimed pages
|
|
|
|
pgactivate
|
|
|
|
Amount of pages moved to the active LRU list
|
|
|
|
pgdeactivate
|
|
|
|
Amount of pages moved to the inactive LRU lis
|
|
|
|
pglazyfree
|
|
|
|
Amount of pages postponed to be freed under memory pressure
|
|
|
|
pglazyfreed
|
|
|
|
Amount of reclaimed lazyfree pages
|
|
|
|
thp_fault_alloc
|
|
|
|
Number of transparent hugepages which were allocated to satisfy
|
|
a page fault, including COW faults. This counter is not present
|
|
when CONFIG_TRANSPARENT_HUGEPAGE is not set.
|
|
|
|
thp_collapse_alloc
|
|
|
|
Number of transparent hugepages which were allocated to allow
|
|
collapsing an existing range of pages. This counter is not
|
|
present when CONFIG_TRANSPARENT_HUGEPAGE is not set.
|
|
|
|
memory.swap.current
|
|
A read-only single value file which exists on non-root
|
|
cgroups.
|
|
|
|
The total amount of swap currently being used by the cgroup
|
|
and its descendants.
|
|
|
|
memory.swap.max
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Swap usage hard limit. If a cgroup's swap usage reaches this
|
|
limit, anonymous memory of the cgroup will not be swapped out.
|
|
|
|
memory.swap.events
|
|
A read-only flat-keyed file which exists on non-root cgroups.
|
|
The following entries are defined. Unless specified
|
|
otherwise, a value change in this file generates a file
|
|
modified event.
|
|
|
|
max
|
|
The number of times the cgroup's swap usage was about
|
|
to go over the max boundary and swap allocation
|
|
failed.
|
|
|
|
fail
|
|
The number of times swap allocation failed either
|
|
because of running out of swap system-wide or max
|
|
limit.
|
|
|
|
When reduced under the current usage, the existing swap
|
|
entries are reclaimed gradually and the swap usage may stay
|
|
higher than the limit for an extended period of time. This
|
|
reduces the impact on the workload and memory management.
|
|
|
|
memory.pressure
|
|
A read-only nested-key file which exists on non-root cgroups.
|
|
|
|
Shows pressure stall information for memory. See
|
|
Documentation/accounting/psi.rst for details.
|
|
|
|
|
|
Usage Guidelines
|
|
~~~~~~~~~~~~~~~~
|
|
|
|
"memory.high" is the main mechanism to control memory usage.
|
|
Over-committing on high limit (sum of high limits > available memory)
|
|
and letting global memory pressure to distribute memory according to
|
|
usage is a viable strategy.
|
|
|
|
Because breach of the high limit doesn't trigger the OOM killer but
|
|
throttles the offending cgroup, a management agent has ample
|
|
opportunities to monitor and take appropriate actions such as granting
|
|
more memory or terminating the workload.
|
|
|
|
Determining whether a cgroup has enough memory is not trivial as
|
|
memory usage doesn't indicate whether the workload can benefit from
|
|
more memory. For example, a workload which writes data received from
|
|
network to a file can use all available memory but can also operate as
|
|
performant with a small amount of memory. A measure of memory
|
|
pressure - how much the workload is being impacted due to lack of
|
|
memory - is necessary to determine whether a workload needs more
|
|
memory; unfortunately, memory pressure monitoring mechanism isn't
|
|
implemented yet.
|
|
|
|
|
|
Memory Ownership
|
|
~~~~~~~~~~~~~~~~
|
|
|
|
A memory area is charged to the cgroup which instantiated it and stays
|
|
charged to the cgroup until the area is released. Migrating a process
|
|
to a different cgroup doesn't move the memory usages that it
|
|
instantiated while in the previous cgroup to the new cgroup.
|
|
|
|
A memory area may be used by processes belonging to different cgroups.
|
|
To which cgroup the area will be charged is in-deterministic; however,
|
|
over time, the memory area is likely to end up in a cgroup which has
|
|
enough memory allowance to avoid high reclaim pressure.
|
|
|
|
If a cgroup sweeps a considerable amount of memory which is expected
|
|
to be accessed repeatedly by other cgroups, it may make sense to use
|
|
POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
|
|
belonging to the affected files to ensure correct memory ownership.
|
|
|
|
|
|
IO
|
|
--
|
|
|
|
The "io" controller regulates the distribution of IO resources. This
|
|
controller implements both weight based and absolute bandwidth or IOPS
|
|
limit distribution; however, weight based distribution is available
|
|
only if cfq-iosched is in use and neither scheme is available for
|
|
blk-mq devices.
|
|
|
|
|
|
IO Interface Files
|
|
~~~~~~~~~~~~~~~~~~
|
|
|
|
io.stat
|
|
A read-only nested-keyed file which exists on non-root
|
|
cgroups.
|
|
|
|
Lines are keyed by $MAJ:$MIN device numbers and not ordered.
|
|
The following nested keys are defined.
|
|
|
|
====== =====================
|
|
rbytes Bytes read
|
|
wbytes Bytes written
|
|
rios Number of read IOs
|
|
wios Number of write IOs
|
|
dbytes Bytes discarded
|
|
dios Number of discard IOs
|
|
====== =====================
|
|
|
|
An example read output follows:
|
|
|
|
8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 dbytes=0 dios=0
|
|
8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021
|
|
|
|
io.cost.qos
|
|
A read-write nested-keyed file with exists only on the root
|
|
cgroup.
|
|
|
|
This file configures the Quality of Service of the IO cost
|
|
model based controller (CONFIG_BLK_CGROUP_IOCOST) which
|
|
currently implements "io.weight" proportional control. Lines
|
|
are keyed by $MAJ:$MIN device numbers and not ordered. The
|
|
line for a given device is populated on the first write for
|
|
the device on "io.cost.qos" or "io.cost.model". The following
|
|
nested keys are defined.
|
|
|
|
====== =====================================
|
|
enable Weight-based control enable
|
|
ctrl "auto" or "user"
|
|
rpct Read latency percentile [0, 100]
|
|
rlat Read latency threshold
|
|
wpct Write latency percentile [0, 100]
|
|
wlat Write latency threshold
|
|
min Minimum scaling percentage [1, 10000]
|
|
max Maximum scaling percentage [1, 10000]
|
|
====== =====================================
|
|
|
|
The controller is disabled by default and can be enabled by
|
|
setting "enable" to 1. "rpct" and "wpct" parameters default
|
|
to zero and the controller uses internal device saturation
|
|
state to adjust the overall IO rate between "min" and "max".
|
|
|
|
When a better control quality is needed, latency QoS
|
|
parameters can be configured. For example::
|
|
|
|
8:16 enable=1 ctrl=auto rpct=95.00 rlat=75000 wpct=95.00 wlat=150000 min=50.00 max=150.0
|
|
|
|
shows that on sdb, the controller is enabled, will consider
|
|
the device saturated if the 95th percentile of read completion
|
|
latencies is above 75ms or write 150ms, and adjust the overall
|
|
IO issue rate between 50% and 150% accordingly.
|
|
|
|
The lower the saturation point, the better the latency QoS at
|
|
the cost of aggregate bandwidth. The narrower the allowed
|
|
adjustment range between "min" and "max", the more conformant
|
|
to the cost model the IO behavior. Note that the IO issue
|
|
base rate may be far off from 100% and setting "min" and "max"
|
|
blindly can lead to a significant loss of device capacity or
|
|
control quality. "min" and "max" are useful for regulating
|
|
devices which show wide temporary behavior changes - e.g. a
|
|
ssd which accepts writes at the line speed for a while and
|
|
then completely stalls for multiple seconds.
|
|
|
|
When "ctrl" is "auto", the parameters are controlled by the
|
|
kernel and may change automatically. Setting "ctrl" to "user"
|
|
or setting any of the percentile and latency parameters puts
|
|
it into "user" mode and disables the automatic changes. The
|
|
automatic mode can be restored by setting "ctrl" to "auto".
|
|
|
|
io.cost.model
|
|
A read-write nested-keyed file with exists only on the root
|
|
cgroup.
|
|
|
|
This file configures the cost model of the IO cost model based
|
|
controller (CONFIG_BLK_CGROUP_IOCOST) which currently
|
|
implements "io.weight" proportional control. Lines are keyed
|
|
by $MAJ:$MIN device numbers and not ordered. The line for a
|
|
given device is populated on the first write for the device on
|
|
"io.cost.qos" or "io.cost.model". The following nested keys
|
|
are defined.
|
|
|
|
===== ================================
|
|
ctrl "auto" or "user"
|
|
model The cost model in use - "linear"
|
|
===== ================================
|
|
|
|
When "ctrl" is "auto", the kernel may change all parameters
|
|
dynamically. When "ctrl" is set to "user" or any other
|
|
parameters are written to, "ctrl" become "user" and the
|
|
automatic changes are disabled.
|
|
|
|
When "model" is "linear", the following model parameters are
|
|
defined.
|
|
|
|
============= ========================================
|
|
[r|w]bps The maximum sequential IO throughput
|
|
[r|w]seqiops The maximum 4k sequential IOs per second
|
|
[r|w]randiops The maximum 4k random IOs per second
|
|
============= ========================================
|
|
|
|
From the above, the builtin linear model determines the base
|
|
costs of a sequential and random IO and the cost coefficient
|
|
for the IO size. While simple, this model can cover most
|
|
common device classes acceptably.
|
|
|
|
The IO cost model isn't expected to be accurate in absolute
|
|
sense and is scaled to the device behavior dynamically.
|
|
|
|
If needed, tools/cgroup/iocost_coef_gen.py can be used to
|
|
generate device-specific coefficients.
|
|
|
|
io.weight
|
|
A read-write flat-keyed file which exists on non-root cgroups.
|
|
The default is "default 100".
|
|
|
|
The first line is the default weight applied to devices
|
|
without specific override. The rest are overrides keyed by
|
|
$MAJ:$MIN device numbers and not ordered. The weights are in
|
|
the range [1, 10000] and specifies the relative amount IO time
|
|
the cgroup can use in relation to its siblings.
|
|
|
|
The default weight can be updated by writing either "default
|
|
$WEIGHT" or simply "$WEIGHT". Overrides can be set by writing
|
|
"$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default".
|
|
|
|
An example read output follows::
|
|
|
|
default 100
|
|
8:16 200
|
|
8:0 50
|
|
|
|
io.max
|
|
A read-write nested-keyed file which exists on non-root
|
|
cgroups.
|
|
|
|
BPS and IOPS based IO limit. Lines are keyed by $MAJ:$MIN
|
|
device numbers and not ordered. The following nested keys are
|
|
defined.
|
|
|
|
===== ==================================
|
|
rbps Max read bytes per second
|
|
wbps Max write bytes per second
|
|
riops Max read IO operations per second
|
|
wiops Max write IO operations per second
|
|
===== ==================================
|
|
|
|
When writing, any number of nested key-value pairs can be
|
|
specified in any order. "max" can be specified as the value
|
|
to remove a specific limit. If the same key is specified
|
|
multiple times, the outcome is undefined.
|
|
|
|
BPS and IOPS are measured in each IO direction and IOs are
|
|
delayed if limit is reached. Temporary bursts are allowed.
|
|
|
|
Setting read limit at 2M BPS and write at 120 IOPS for 8:16::
|
|
|
|
echo "8:16 rbps=2097152 wiops=120" > io.max
|
|
|
|
Reading returns the following::
|
|
|
|
8:16 rbps=2097152 wbps=max riops=max wiops=120
|
|
|
|
Write IOPS limit can be removed by writing the following::
|
|
|
|
echo "8:16 wiops=max" > io.max
|
|
|
|
Reading now returns the following::
|
|
|
|
8:16 rbps=2097152 wbps=max riops=max wiops=max
|
|
|
|
io.pressure
|
|
A read-only nested-key file which exists on non-root cgroups.
|
|
|
|
Shows pressure stall information for IO. See
|
|
Documentation/accounting/psi.rst for details.
|
|
|
|
|
|
Writeback
|
|
~~~~~~~~~
|
|
|
|
Page cache is dirtied through buffered writes and shared mmaps and
|
|
written asynchronously to the backing filesystem by the writeback
|
|
mechanism. Writeback sits between the memory and IO domains and
|
|
regulates the proportion of dirty memory by balancing dirtying and
|
|
write IOs.
|
|
|
|
The io controller, in conjunction with the memory controller,
|
|
implements control of page cache writeback IOs. The memory controller
|
|
defines the memory domain that dirty memory ratio is calculated and
|
|
maintained for and the io controller defines the io domain which
|
|
writes out dirty pages for the memory domain. Both system-wide and
|
|
per-cgroup dirty memory states are examined and the more restrictive
|
|
of the two is enforced.
|
|
|
|
cgroup writeback requires explicit support from the underlying
|
|
filesystem. Currently, cgroup writeback is implemented on ext2, ext4
|
|
and btrfs. On other filesystems, all writeback IOs are attributed to
|
|
the root cgroup.
|
|
|
|
There are inherent differences in memory and writeback management
|
|
which affects how cgroup ownership is tracked. Memory is tracked per
|
|
page while writeback per inode. For the purpose of writeback, an
|
|
inode is assigned to a cgroup and all IO requests to write dirty pages
|
|
from the inode are attributed to that cgroup.
|
|
|
|
As cgroup ownership for memory is tracked per page, there can be pages
|
|
which are associated with different cgroups than the one the inode is
|
|
associated with. These are called foreign pages. The writeback
|
|
constantly keeps track of foreign pages and, if a particular foreign
|
|
cgroup becomes the majority over a certain period of time, switches
|
|
the ownership of the inode to that cgroup.
|
|
|
|
While this model is enough for most use cases where a given inode is
|
|
mostly dirtied by a single cgroup even when the main writing cgroup
|
|
changes over time, use cases where multiple cgroups write to a single
|
|
inode simultaneously are not supported well. In such circumstances, a
|
|
significant portion of IOs are likely to be attributed incorrectly.
|
|
As memory controller assigns page ownership on the first use and
|
|
doesn't update it until the page is released, even if writeback
|
|
strictly follows page ownership, multiple cgroups dirtying overlapping
|
|
areas wouldn't work as expected. It's recommended to avoid such usage
|
|
patterns.
|
|
|
|
The sysctl knobs which affect writeback behavior are applied to cgroup
|
|
writeback as follows.
|
|
|
|
vm.dirty_background_ratio, vm.dirty_ratio
|
|
These ratios apply the same to cgroup writeback with the
|
|
amount of available memory capped by limits imposed by the
|
|
memory controller and system-wide clean memory.
|
|
|
|
vm.dirty_background_bytes, vm.dirty_bytes
|
|
For cgroup writeback, this is calculated into ratio against
|
|
total available memory and applied the same way as
|
|
vm.dirty[_background]_ratio.
|
|
|
|
|
|
IO Latency
|
|
~~~~~~~~~~
|
|
|
|
This is a cgroup v2 controller for IO workload protection. You provide a group
|
|
with a latency target, and if the average latency exceeds that target the
|
|
controller will throttle any peers that have a lower latency target than the
|
|
protected workload.
|
|
|
|
The limits are only applied at the peer level in the hierarchy. This means that
|
|
in the diagram below, only groups A, B, and C will influence each other, and
|
|
groups D and F will influence each other. Group G will influence nobody::
|
|
|
|
[root]
|
|
/ | \
|
|
A B C
|
|
/ \ |
|
|
D F G
|
|
|
|
|
|
So the ideal way to configure this is to set io.latency in groups A, B, and C.
|
|
Generally you do not want to set a value lower than the latency your device
|
|
supports. Experiment to find the value that works best for your workload.
|
|
Start at higher than the expected latency for your device and watch the
|
|
avg_lat value in io.stat for your workload group to get an idea of the
|
|
latency you see during normal operation. Use the avg_lat value as a basis for
|
|
your real setting, setting at 10-15% higher than the value in io.stat.
|
|
|
|
How IO Latency Throttling Works
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
io.latency is work conserving; so as long as everybody is meeting their latency
|
|
target the controller doesn't do anything. Once a group starts missing its
|
|
target it begins throttling any peer group that has a higher target than itself.
|
|
This throttling takes 2 forms:
|
|
|
|
- Queue depth throttling. This is the number of outstanding IO's a group is
|
|
allowed to have. We will clamp down relatively quickly, starting at no limit
|
|
and going all the way down to 1 IO at a time.
|
|
|
|
- Artificial delay induction. There are certain types of IO that cannot be
|
|
throttled without possibly adversely affecting higher priority groups. This
|
|
includes swapping and metadata IO. These types of IO are allowed to occur
|
|
normally, however they are "charged" to the originating group. If the
|
|
originating group is being throttled you will see the use_delay and delay
|
|
fields in io.stat increase. The delay value is how many microseconds that are
|
|
being added to any process that runs in this group. Because this number can
|
|
grow quite large if there is a lot of swapping or metadata IO occurring we
|
|
limit the individual delay events to 1 second at a time.
|
|
|
|
Once the victimized group starts meeting its latency target again it will start
|
|
unthrottling any peer groups that were throttled previously. If the victimized
|
|
group simply stops doing IO the global counter will unthrottle appropriately.
|
|
|
|
IO Latency Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
io.latency
|
|
This takes a similar format as the other controllers.
|
|
|
|
"MAJOR:MINOR target=<target time in microseconds"
|
|
|
|
io.stat
|
|
If the controller is enabled you will see extra stats in io.stat in
|
|
addition to the normal ones.
|
|
|
|
depth
|
|
This is the current queue depth for the group.
|
|
|
|
avg_lat
|
|
This is an exponential moving average with a decay rate of 1/exp
|
|
bound by the sampling interval. The decay rate interval can be
|
|
calculated by multiplying the win value in io.stat by the
|
|
corresponding number of samples based on the win value.
|
|
|
|
win
|
|
The sampling window size in milliseconds. This is the minimum
|
|
duration of time between evaluation events. Windows only elapse
|
|
with IO activity. Idle periods extend the most recent window.
|
|
|
|
PID
|
|
---
|
|
|
|
The process number controller is used to allow a cgroup to stop any
|
|
new tasks from being fork()'d or clone()'d after a specified limit is
|
|
reached.
|
|
|
|
The number of tasks in a cgroup can be exhausted in ways which other
|
|
controllers cannot prevent, thus warranting its own controller. For
|
|
example, a fork bomb is likely to exhaust the number of tasks before
|
|
hitting memory restrictions.
|
|
|
|
Note that PIDs used in this controller refer to TIDs, process IDs as
|
|
used by the kernel.
|
|
|
|
|
|
PID Interface Files
|
|
~~~~~~~~~~~~~~~~~~~
|
|
|
|
pids.max
|
|
A read-write single value file which exists on non-root
|
|
cgroups. The default is "max".
|
|
|
|
Hard limit of number of processes.
|
|
|
|
pids.current
|
|
A read-only single value file which exists on all cgroups.
|
|
|
|
The number of processes currently in the cgroup and its
|
|
descendants.
|
|
|
|
Organisational operations are not blocked by cgroup policies, so it is
|
|
possible to have pids.current > pids.max. This can be done by either
|
|
setting the limit to be smaller than pids.current, or attaching enough
|
|
processes to the cgroup such that pids.current is larger than
|
|
pids.max. However, it is not possible to violate a cgroup PID policy
|
|
through fork() or clone(). These will return -EAGAIN if the creation
|
|
of a new process would cause a cgroup policy to be violated.
|
|
|
|
|
|
Cpuset
|
|
------
|
|
|
|
The "cpuset" controller provides a mechanism for constraining
|
|
the CPU and memory node placement of tasks to only the resources
|
|
specified in the cpuset interface files in a task's current cgroup.
|
|
This is especially valuable on large NUMA systems where placing jobs
|
|
on properly sized subsets of the systems with careful processor and
|
|
memory placement to reduce cross-node memory access and contention
|
|
can improve overall system performance.
|
|
|
|
The "cpuset" controller is hierarchical. That means the controller
|
|
cannot use CPUs or memory nodes not allowed in its parent.
|
|
|
|
|
|
Cpuset Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
cpuset.cpus
|
|
A read-write multiple values file which exists on non-root
|
|
cpuset-enabled cgroups.
|
|
|
|
It lists the requested CPUs to be used by tasks within this
|
|
cgroup. The actual list of CPUs to be granted, however, is
|
|
subjected to constraints imposed by its parent and can differ
|
|
from the requested CPUs.
|
|
|
|
The CPU numbers are comma-separated numbers or ranges.
|
|
For example:
|
|
|
|
# cat cpuset.cpus
|
|
0-4,6,8-10
|
|
|
|
An empty value indicates that the cgroup is using the same
|
|
setting as the nearest cgroup ancestor with a non-empty
|
|
"cpuset.cpus" or all the available CPUs if none is found.
|
|
|
|
The value of "cpuset.cpus" stays constant until the next update
|
|
and won't be affected by any CPU hotplug events.
|
|
|
|
cpuset.cpus.effective
|
|
A read-only multiple values file which exists on all
|
|
cpuset-enabled cgroups.
|
|
|
|
It lists the onlined CPUs that are actually granted to this
|
|
cgroup by its parent. These CPUs are allowed to be used by
|
|
tasks within the current cgroup.
|
|
|
|
If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows
|
|
all the CPUs from the parent cgroup that can be available to
|
|
be used by this cgroup. Otherwise, it should be a subset of
|
|
"cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus"
|
|
can be granted. In this case, it will be treated just like an
|
|
empty "cpuset.cpus".
|
|
|
|
Its value will be affected by CPU hotplug events.
|
|
|
|
cpuset.mems
|
|
A read-write multiple values file which exists on non-root
|
|
cpuset-enabled cgroups.
|
|
|
|
It lists the requested memory nodes to be used by tasks within
|
|
this cgroup. The actual list of memory nodes granted, however,
|
|
is subjected to constraints imposed by its parent and can differ
|
|
from the requested memory nodes.
|
|
|
|
The memory node numbers are comma-separated numbers or ranges.
|
|
For example:
|
|
|
|
# cat cpuset.mems
|
|
0-1,3
|
|
|
|
An empty value indicates that the cgroup is using the same
|
|
setting as the nearest cgroup ancestor with a non-empty
|
|
"cpuset.mems" or all the available memory nodes if none
|
|
is found.
|
|
|
|
The value of "cpuset.mems" stays constant until the next update
|
|
and won't be affected by any memory nodes hotplug events.
|
|
|
|
cpuset.mems.effective
|
|
A read-only multiple values file which exists on all
|
|
cpuset-enabled cgroups.
|
|
|
|
It lists the onlined memory nodes that are actually granted to
|
|
this cgroup by its parent. These memory nodes are allowed to
|
|
be used by tasks within the current cgroup.
|
|
|
|
If "cpuset.mems" is empty, it shows all the memory nodes from the
|
|
parent cgroup that will be available to be used by this cgroup.
|
|
Otherwise, it should be a subset of "cpuset.mems" unless none of
|
|
the memory nodes listed in "cpuset.mems" can be granted. In this
|
|
case, it will be treated just like an empty "cpuset.mems".
|
|
|
|
Its value will be affected by memory nodes hotplug events.
|
|
|
|
cpuset.cpus.partition
|
|
A read-write single value file which exists on non-root
|
|
cpuset-enabled cgroups. This flag is owned by the parent cgroup
|
|
and is not delegatable.
|
|
|
|
It accepts only the following input values when written to.
|
|
|
|
"root" - a paritition root
|
|
"member" - a non-root member of a partition
|
|
|
|
When set to be a partition root, the current cgroup is the
|
|
root of a new partition or scheduling domain that comprises
|
|
itself and all its descendants except those that are separate
|
|
partition roots themselves and their descendants. The root
|
|
cgroup is always a partition root.
|
|
|
|
There are constraints on where a partition root can be set.
|
|
It can only be set in a cgroup if all the following conditions
|
|
are true.
|
|
|
|
1) The "cpuset.cpus" is not empty and the list of CPUs are
|
|
exclusive, i.e. they are not shared by any of its siblings.
|
|
2) The parent cgroup is a partition root.
|
|
3) The "cpuset.cpus" is also a proper subset of the parent's
|
|
"cpuset.cpus.effective".
|
|
4) There is no child cgroups with cpuset enabled. This is for
|
|
eliminating corner cases that have to be handled if such a
|
|
condition is allowed.
|
|
|
|
Setting it to partition root will take the CPUs away from the
|
|
effective CPUs of the parent cgroup. Once it is set, this
|
|
file cannot be reverted back to "member" if there are any child
|
|
cgroups with cpuset enabled.
|
|
|
|
A parent partition cannot distribute all its CPUs to its
|
|
child partitions. There must be at least one cpu left in the
|
|
parent partition.
|
|
|
|
Once becoming a partition root, changes to "cpuset.cpus" is
|
|
generally allowed as long as the first condition above is true,
|
|
the change will not take away all the CPUs from the parent
|
|
partition and the new "cpuset.cpus" value is a superset of its
|
|
children's "cpuset.cpus" values.
|
|
|
|
Sometimes, external factors like changes to ancestors'
|
|
"cpuset.cpus" or cpu hotplug can cause the state of the partition
|
|
root to change. On read, the "cpuset.sched.partition" file
|
|
can show the following values.
|
|
|
|
"member" Non-root member of a partition
|
|
"root" Partition root
|
|
"root invalid" Invalid partition root
|
|
|
|
It is a partition root if the first 2 partition root conditions
|
|
above are true and at least one CPU from "cpuset.cpus" is
|
|
granted by the parent cgroup.
|
|
|
|
A partition root can become invalid if none of CPUs requested
|
|
in "cpuset.cpus" can be granted by the parent cgroup or the
|
|
parent cgroup is no longer a partition root itself. In this
|
|
case, it is not a real partition even though the restriction
|
|
of the first partition root condition above will still apply.
|
|
The cpu affinity of all the tasks in the cgroup will then be
|
|
associated with CPUs in the nearest ancestor partition.
|
|
|
|
An invalid partition root can be transitioned back to a
|
|
real partition root if at least one of the requested CPUs
|
|
can now be granted by its parent. In this case, the cpu
|
|
affinity of all the tasks in the formerly invalid partition
|
|
will be associated to the CPUs of the newly formed partition.
|
|
Changing the partition state of an invalid partition root to
|
|
"member" is always allowed even if child cpusets are present.
|
|
|
|
|
|
Device controller
|
|
-----------------
|
|
|
|
Device controller manages access to device files. It includes both
|
|
creation of new device files (using mknod), and access to the
|
|
existing device files.
|
|
|
|
Cgroup v2 device controller has no interface files and is implemented
|
|
on top of cgroup BPF. To control access to device files, a user may
|
|
create bpf programs of the BPF_CGROUP_DEVICE type and attach them
|
|
to cgroups. On an attempt to access a device file, corresponding
|
|
BPF programs will be executed, and depending on the return value
|
|
the attempt will succeed or fail with -EPERM.
|
|
|
|
A BPF_CGROUP_DEVICE program takes a pointer to the bpf_cgroup_dev_ctx
|
|
structure, which describes the device access attempt: access type
|
|
(mknod/read/write) and device (type, major and minor numbers).
|
|
If the program returns 0, the attempt fails with -EPERM, otherwise
|
|
it succeeds.
|
|
|
|
An example of BPF_CGROUP_DEVICE program may be found in the kernel
|
|
source tree in the tools/testing/selftests/bpf/dev_cgroup.c file.
|
|
|
|
|
|
RDMA
|
|
----
|
|
|
|
The "rdma" controller regulates the distribution and accounting of
|
|
of RDMA resources.
|
|
|
|
RDMA Interface Files
|
|
~~~~~~~~~~~~~~~~~~~~
|
|
|
|
rdma.max
|
|
A readwrite nested-keyed file that exists for all the cgroups
|
|
except root that describes current configured resource limit
|
|
for a RDMA/IB device.
|
|
|
|
Lines are keyed by device name and are not ordered.
|
|
Each line contains space separated resource name and its configured
|
|
limit that can be distributed.
|
|
|
|
The following nested keys are defined.
|
|
|
|
========== =============================
|
|
hca_handle Maximum number of HCA Handles
|
|
hca_object Maximum number of HCA Objects
|
|
========== =============================
|
|
|
|
An example for mlx4 and ocrdma device follows::
|
|
|
|
mlx4_0 hca_handle=2 hca_object=2000
|
|
ocrdma1 hca_handle=3 hca_object=max
|
|
|
|
rdma.current
|
|
A read-only file that describes current resource usage.
|
|
It exists for all the cgroup except root.
|
|
|
|
An example for mlx4 and ocrdma device follows::
|
|
|
|
mlx4_0 hca_handle=1 hca_object=20
|
|
ocrdma1 hca_handle=1 hca_object=23
|
|
|
|
|
|
Misc
|
|
----
|
|
|
|
perf_event
|
|
~~~~~~~~~~
|
|
|
|
perf_event controller, if not mounted on a legacy hierarchy, is
|
|
automatically enabled on the v2 hierarchy so that perf events can
|
|
always be filtered by cgroup v2 path. The controller can still be
|
|
moved to a legacy hierarchy after v2 hierarchy is populated.
|
|
|
|
|
|
Non-normative information
|
|
-------------------------
|
|
|
|
This section contains information that isn't considered to be a part of
|
|
the stable kernel API and so is subject to change.
|
|
|
|
|
|
CPU controller root cgroup process behaviour
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
When distributing CPU cycles in the root cgroup each thread in this
|
|
cgroup is treated as if it was hosted in a separate child cgroup of the
|
|
root cgroup. This child cgroup weight is dependent on its thread nice
|
|
level.
|
|
|
|
For details of this mapping see sched_prio_to_weight array in
|
|
kernel/sched/core.c file (values from this array should be scaled
|
|
appropriately so the neutral - nice 0 - value is 100 instead of 1024).
|
|
|
|
|
|
IO controller root cgroup process behaviour
|
|
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
|
|
|
Root cgroup processes are hosted in an implicit leaf child node.
|
|
When distributing IO resources this implicit child node is taken into
|
|
account as if it was a normal child cgroup of the root cgroup with a
|
|
weight value of 200.
|
|
|
|
|
|
Namespace
|
|
=========
|
|
|
|
Basics
|
|
------
|
|
|
|
cgroup namespace provides a mechanism to virtualize the view of the
|
|
"/proc/$PID/cgroup" file and cgroup mounts. The CLONE_NEWCGROUP clone
|
|
flag can be used with clone(2) and unshare(2) to create a new cgroup
|
|
namespace. The process running inside the cgroup namespace will have
|
|
its "/proc/$PID/cgroup" output restricted to cgroupns root. The
|
|
cgroupns root is the cgroup of the process at the time of creation of
|
|
the cgroup namespace.
|
|
|
|
Without cgroup namespace, the "/proc/$PID/cgroup" file shows the
|
|
complete path of the cgroup of a process. In a container setup where
|
|
a set of cgroups and namespaces are intended to isolate processes the
|
|
"/proc/$PID/cgroup" file may leak potential system level information
|
|
to the isolated processes. For Example::
|
|
|
|
# cat /proc/self/cgroup
|
|
0::/batchjobs/container_id1
|
|
|
|
The path '/batchjobs/container_id1' can be considered as system-data
|
|
and undesirable to expose to the isolated processes. cgroup namespace
|
|
can be used to restrict visibility of this path. For example, before
|
|
creating a cgroup namespace, one would see::
|
|
|
|
# ls -l /proc/self/ns/cgroup
|
|
lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835]
|
|
# cat /proc/self/cgroup
|
|
0::/batchjobs/container_id1
|
|
|
|
After unsharing a new namespace, the view changes::
|
|
|
|
# ls -l /proc/self/ns/cgroup
|
|
lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183]
|
|
# cat /proc/self/cgroup
|
|
0::/
|
|
|
|
When some thread from a multi-threaded process unshares its cgroup
|
|
namespace, the new cgroupns gets applied to the entire process (all
|
|
the threads). This is natural for the v2 hierarchy; however, for the
|
|
legacy hierarchies, this may be unexpected.
|
|
|
|
A cgroup namespace is alive as long as there are processes inside or
|
|
mounts pinning it. When the last usage goes away, the cgroup
|
|
namespace is destroyed. The cgroupns root and the actual cgroups
|
|
remain.
|
|
|
|
|
|
The Root and Views
|
|
------------------
|
|
|
|
The 'cgroupns root' for a cgroup namespace is the cgroup in which the
|
|
process calling unshare(2) is running. For example, if a process in
|
|
/batchjobs/container_id1 cgroup calls unshare, cgroup
|
|
/batchjobs/container_id1 becomes the cgroupns root. For the
|
|
init_cgroup_ns, this is the real root ('/') cgroup.
|
|
|
|
The cgroupns root cgroup does not change even if the namespace creator
|
|
process later moves to a different cgroup::
|
|
|
|
# ~/unshare -c # unshare cgroupns in some cgroup
|
|
# cat /proc/self/cgroup
|
|
0::/
|
|
# mkdir sub_cgrp_1
|
|
# echo 0 > sub_cgrp_1/cgroup.procs
|
|
# cat /proc/self/cgroup
|
|
0::/sub_cgrp_1
|
|
|
|
Each process gets its namespace-specific view of "/proc/$PID/cgroup"
|
|
|
|
Processes running inside the cgroup namespace will be able to see
|
|
cgroup paths (in /proc/self/cgroup) only inside their root cgroup.
|
|
From within an unshared cgroupns::
|
|
|
|
# sleep 100000 &
|
|
[1] 7353
|
|
# echo 7353 > sub_cgrp_1/cgroup.procs
|
|
# cat /proc/7353/cgroup
|
|
0::/sub_cgrp_1
|
|
|
|
From the initial cgroup namespace, the real cgroup path will be
|
|
visible::
|
|
|
|
$ cat /proc/7353/cgroup
|
|
0::/batchjobs/container_id1/sub_cgrp_1
|
|
|
|
From a sibling cgroup namespace (that is, a namespace rooted at a
|
|
different cgroup), the cgroup path relative to its own cgroup
|
|
namespace root will be shown. For instance, if PID 7353's cgroup
|
|
namespace root is at '/batchjobs/container_id2', then it will see::
|
|
|
|
# cat /proc/7353/cgroup
|
|
0::/../container_id2/sub_cgrp_1
|
|
|
|
Note that the relative path always starts with '/' to indicate that
|
|
its relative to the cgroup namespace root of the caller.
|
|
|
|
|
|
Migration and setns(2)
|
|
----------------------
|
|
|
|
Processes inside a cgroup namespace can move into and out of the
|
|
namespace root if they have proper access to external cgroups. For
|
|
example, from inside a namespace with cgroupns root at
|
|
/batchjobs/container_id1, and assuming that the global hierarchy is
|
|
still accessible inside cgroupns::
|
|
|
|
# cat /proc/7353/cgroup
|
|
0::/sub_cgrp_1
|
|
# echo 7353 > batchjobs/container_id2/cgroup.procs
|
|
# cat /proc/7353/cgroup
|
|
0::/../container_id2
|
|
|
|
Note that this kind of setup is not encouraged. A task inside cgroup
|
|
namespace should only be exposed to its own cgroupns hierarchy.
|
|
|
|
setns(2) to another cgroup namespace is allowed when:
|
|
|
|
(a) the process has CAP_SYS_ADMIN against its current user namespace
|
|
(b) the process has CAP_SYS_ADMIN against the target cgroup
|
|
namespace's userns
|
|
|
|
No implicit cgroup changes happen with attaching to another cgroup
|
|
namespace. It is expected that the someone moves the attaching
|
|
process under the target cgroup namespace root.
|
|
|
|
|
|
Interaction with Other Namespaces
|
|
---------------------------------
|
|
|
|
Namespace specific cgroup hierarchy can be mounted by a process
|
|
running inside a non-init cgroup namespace::
|
|
|
|
# mount -t cgroup2 none $MOUNT_POINT
|
|
|
|
This will mount the unified cgroup hierarchy with cgroupns root as the
|
|
filesystem root. The process needs CAP_SYS_ADMIN against its user and
|
|
mount namespaces.
|
|
|
|
The virtualization of /proc/self/cgroup file combined with restricting
|
|
the view of cgroup hierarchy by namespace-private cgroupfs mount
|
|
provides a properly isolated cgroup view inside the container.
|
|
|
|
|
|
Information on Kernel Programming
|
|
=================================
|
|
|
|
This section contains kernel programming information in the areas
|
|
where interacting with cgroup is necessary. cgroup core and
|
|
controllers are not covered.
|
|
|
|
|
|
Filesystem Support for Writeback
|
|
--------------------------------
|
|
|
|
A filesystem can support cgroup writeback by updating
|
|
address_space_operations->writepage[s]() to annotate bio's using the
|
|
following two functions.
|
|
|
|
wbc_init_bio(@wbc, @bio)
|
|
Should be called for each bio carrying writeback data and
|
|
associates the bio with the inode's owner cgroup and the
|
|
corresponding request queue. This must be called after
|
|
a queue (device) has been associated with the bio and
|
|
before submission.
|
|
|
|
wbc_account_cgroup_owner(@wbc, @page, @bytes)
|
|
Should be called for each data segment being written out.
|
|
While this function doesn't care exactly when it's called
|
|
during the writeback session, it's the easiest and most
|
|
natural to call it as data segments are added to a bio.
|
|
|
|
With writeback bio's annotated, cgroup support can be enabled per
|
|
super_block by setting SB_I_CGROUPWB in ->s_iflags. This allows for
|
|
selective disabling of cgroup writeback support which is helpful when
|
|
certain filesystem features, e.g. journaled data mode, are
|
|
incompatible.
|
|
|
|
wbc_init_bio() binds the specified bio to its cgroup. Depending on
|
|
the configuration, the bio may be executed at a lower priority and if
|
|
the writeback session is holding shared resources, e.g. a journal
|
|
entry, may lead to priority inversion. There is no one easy solution
|
|
for the problem. Filesystems can try to work around specific problem
|
|
cases by skipping wbc_init_bio() and using bio_associate_blkg()
|
|
directly.
|
|
|
|
|
|
Deprecated v1 Core Features
|
|
===========================
|
|
|
|
- Multiple hierarchies including named ones are not supported.
|
|
|
|
- All v1 mount options are not supported.
|
|
|
|
- The "tasks" file is removed and "cgroup.procs" is not sorted.
|
|
|
|
- "cgroup.clone_children" is removed.
|
|
|
|
- /proc/cgroups is meaningless for v2. Use "cgroup.controllers" file
|
|
at the root instead.
|
|
|
|
|
|
Issues with v1 and Rationales for v2
|
|
====================================
|
|
|
|
Multiple Hierarchies
|
|
--------------------
|
|
|
|
cgroup v1 allowed an arbitrary number of hierarchies and each
|
|
hierarchy could host any number of controllers. While this seemed to
|
|
provide a high level of flexibility, it wasn't useful in practice.
|
|
|
|
For example, as there is only one instance of each controller, utility
|
|
type controllers such as freezer which can be useful in all
|
|
hierarchies could only be used in one. The issue is exacerbated by
|
|
the fact that controllers couldn't be moved to another hierarchy once
|
|
hierarchies were populated. Another issue was that all controllers
|
|
bound to a hierarchy were forced to have exactly the same view of the
|
|
hierarchy. It wasn't possible to vary the granularity depending on
|
|
the specific controller.
|
|
|
|
In practice, these issues heavily limited which controllers could be
|
|
put on the same hierarchy and most configurations resorted to putting
|
|
each controller on its own hierarchy. Only closely related ones, such
|
|
as the cpu and cpuacct controllers, made sense to be put on the same
|
|
hierarchy. This often meant that userland ended up managing multiple
|
|
similar hierarchies repeating the same steps on each hierarchy
|
|
whenever a hierarchy management operation was necessary.
|
|
|
|
Furthermore, support for multiple hierarchies came at a steep cost.
|
|
It greatly complicated cgroup core implementation but more importantly
|
|
the support for multiple hierarchies restricted how cgroup could be
|
|
used in general and what controllers was able to do.
|
|
|
|
There was no limit on how many hierarchies there might be, which meant
|
|
that a thread's cgroup membership couldn't be described in finite
|
|
length. The key might contain any number of entries and was unlimited
|
|
in length, which made it highly awkward to manipulate and led to
|
|
addition of controllers which existed only to identify membership,
|
|
which in turn exacerbated the original problem of proliferating number
|
|
of hierarchies.
|
|
|
|
Also, as a controller couldn't have any expectation regarding the
|
|
topologies of hierarchies other controllers might be on, each
|
|
controller had to assume that all other controllers were attached to
|
|
completely orthogonal hierarchies. This made it impossible, or at
|
|
least very cumbersome, for controllers to cooperate with each other.
|
|
|
|
In most use cases, putting controllers on hierarchies which are
|
|
completely orthogonal to each other isn't necessary. What usually is
|
|
called for is the ability to have differing levels of granularity
|
|
depending on the specific controller. In other words, hierarchy may
|
|
be collapsed from leaf towards root when viewed from specific
|
|
controllers. For example, a given configuration might not care about
|
|
how memory is distributed beyond a certain level while still wanting
|
|
to control how CPU cycles are distributed.
|
|
|
|
|
|
Thread Granularity
|
|
------------------
|
|
|
|
cgroup v1 allowed threads of a process to belong to different cgroups.
|
|
This didn't make sense for some controllers and those controllers
|
|
ended up implementing different ways to ignore such situations but
|
|
much more importantly it blurred the line between API exposed to
|
|
individual applications and system management interface.
|
|
|
|
Generally, in-process knowledge is available only to the process
|
|
itself; thus, unlike service-level organization of processes,
|
|
categorizing threads of a process requires active participation from
|
|
the application which owns the target process.
|
|
|
|
cgroup v1 had an ambiguously defined delegation model which got abused
|
|
in combination with thread granularity. cgroups were delegated to
|
|
individual applications so that they can create and manage their own
|
|
sub-hierarchies and control resource distributions along them. This
|
|
effectively raised cgroup to the status of a syscall-like API exposed
|
|
to lay programs.
|
|
|
|
First of all, cgroup has a fundamentally inadequate interface to be
|
|
exposed this way. For a process to access its own knobs, it has to
|
|
extract the path on the target hierarchy from /proc/self/cgroup,
|
|
construct the path by appending the name of the knob to the path, open
|
|
and then read and/or write to it. This is not only extremely clunky
|
|
and unusual but also inherently racy. There is no conventional way to
|
|
define transaction across the required steps and nothing can guarantee
|
|
that the process would actually be operating on its own sub-hierarchy.
|
|
|
|
cgroup controllers implemented a number of knobs which would never be
|
|
accepted as public APIs because they were just adding control knobs to
|
|
system-management pseudo filesystem. cgroup ended up with interface
|
|
knobs which were not properly abstracted or refined and directly
|
|
revealed kernel internal details. These knobs got exposed to
|
|
individual applications through the ill-defined delegation mechanism
|
|
effectively abusing cgroup as a shortcut to implementing public APIs
|
|
without going through the required scrutiny.
|
|
|
|
This was painful for both userland and kernel. Userland ended up with
|
|
misbehaving and poorly abstracted interfaces and kernel exposing and
|
|
locked into constructs inadvertently.
|
|
|
|
|
|
Competition Between Inner Nodes and Threads
|
|
-------------------------------------------
|
|
|
|
cgroup v1 allowed threads to be in any cgroups which created an
|
|
interesting problem where threads belonging to a parent cgroup and its
|
|
children cgroups competed for resources. This was nasty as two
|
|
different types of entities competed and there was no obvious way to
|
|
settle it. Different controllers did different things.
|
|
|
|
The cpu controller considered threads and cgroups as equivalents and
|
|
mapped nice levels to cgroup weights. This worked for some cases but
|
|
fell flat when children wanted to be allocated specific ratios of CPU
|
|
cycles and the number of internal threads fluctuated - the ratios
|
|
constantly changed as the number of competing entities fluctuated.
|
|
There also were other issues. The mapping from nice level to weight
|
|
wasn't obvious or universal, and there were various other knobs which
|
|
simply weren't available for threads.
|
|
|
|
The io controller implicitly created a hidden leaf node for each
|
|
cgroup to host the threads. The hidden leaf had its own copies of all
|
|
the knobs with ``leaf_`` prefixed. While this allowed equivalent
|
|
control over internal threads, it was with serious drawbacks. It
|
|
always added an extra layer of nesting which wouldn't be necessary
|
|
otherwise, made the interface messy and significantly complicated the
|
|
implementation.
|
|
|
|
The memory controller didn't have a way to control what happened
|
|
between internal tasks and child cgroups and the behavior was not
|
|
clearly defined. There were attempts to add ad-hoc behaviors and
|
|
knobs to tailor the behavior to specific workloads which would have
|
|
led to problems extremely difficult to resolve in the long term.
|
|
|
|
Multiple controllers struggled with internal tasks and came up with
|
|
different ways to deal with it; unfortunately, all the approaches were
|
|
severely flawed and, furthermore, the widely different behaviors
|
|
made cgroup as a whole highly inconsistent.
|
|
|
|
This clearly is a problem which needs to be addressed from cgroup core
|
|
in a uniform way.
|
|
|
|
|
|
Other Interface Issues
|
|
----------------------
|
|
|
|
cgroup v1 grew without oversight and developed a large number of
|
|
idiosyncrasies and inconsistencies. One issue on the cgroup core side
|
|
was how an empty cgroup was notified - a userland helper binary was
|
|
forked and executed for each event. The event delivery wasn't
|
|
recursive or delegatable. The limitations of the mechanism also led
|
|
to in-kernel event delivery filtering mechanism further complicating
|
|
the interface.
|
|
|
|
Controller interfaces were problematic too. An extreme example is
|
|
controllers completely ignoring hierarchical organization and treating
|
|
all cgroups as if they were all located directly under the root
|
|
cgroup. Some controllers exposed a large amount of inconsistent
|
|
implementation details to userland.
|
|
|
|
There also was no consistency across controllers. When a new cgroup
|
|
was created, some controllers defaulted to not imposing extra
|
|
restrictions while others disallowed any resource usage until
|
|
explicitly configured. Configuration knobs for the same type of
|
|
control used widely differing naming schemes and formats. Statistics
|
|
and information knobs were named arbitrarily and used different
|
|
formats and units even in the same controller.
|
|
|
|
cgroup v2 establishes common conventions where appropriate and updates
|
|
controllers so that they expose minimal and consistent interfaces.
|
|
|
|
|
|
Controller Issues and Remedies
|
|
------------------------------
|
|
|
|
Memory
|
|
~~~~~~
|
|
|
|
The original lower boundary, the soft limit, is defined as a limit
|
|
that is per default unset. As a result, the set of cgroups that
|
|
global reclaim prefers is opt-in, rather than opt-out. The costs for
|
|
optimizing these mostly negative lookups are so high that the
|
|
implementation, despite its enormous size, does not even provide the
|
|
basic desirable behavior. First off, the soft limit has no
|
|
hierarchical meaning. All configured groups are organized in a global
|
|
rbtree and treated like equal peers, regardless where they are located
|
|
in the hierarchy. This makes subtree delegation impossible. Second,
|
|
the soft limit reclaim pass is so aggressive that it not just
|
|
introduces high allocation latencies into the system, but also impacts
|
|
system performance due to overreclaim, to the point where the feature
|
|
becomes self-defeating.
|
|
|
|
The memory.low boundary on the other hand is a top-down allocated
|
|
reserve. A cgroup enjoys reclaim protection when it's within its
|
|
effective low, which makes delegation of subtrees possible. It also
|
|
enjoys having reclaim pressure proportional to its overage when
|
|
above its effective low.
|
|
|
|
The original high boundary, the hard limit, is defined as a strict
|
|
limit that can not budge, even if the OOM killer has to be called.
|
|
But this generally goes against the goal of making the most out of the
|
|
available memory. The memory consumption of workloads varies during
|
|
runtime, and that requires users to overcommit. But doing that with a
|
|
strict upper limit requires either a fairly accurate prediction of the
|
|
working set size or adding slack to the limit. Since working set size
|
|
estimation is hard and error prone, and getting it wrong results in
|
|
OOM kills, most users tend to err on the side of a looser limit and
|
|
end up wasting precious resources.
|
|
|
|
The memory.high boundary on the other hand can be set much more
|
|
conservatively. When hit, it throttles allocations by forcing them
|
|
into direct reclaim to work off the excess, but it never invokes the
|
|
OOM killer. As a result, a high boundary that is chosen too
|
|
aggressively will not terminate the processes, but instead it will
|
|
lead to gradual performance degradation. The user can monitor this
|
|
and make corrections until the minimal memory footprint that still
|
|
gives acceptable performance is found.
|
|
|
|
In extreme cases, with many concurrent allocations and a complete
|
|
breakdown of reclaim progress within the group, the high boundary can
|
|
be exceeded. But even then it's mostly better to satisfy the
|
|
allocation from the slack available in other groups or the rest of the
|
|
system than killing the group. Otherwise, memory.max is there to
|
|
limit this type of spillover and ultimately contain buggy or even
|
|
malicious applications.
|
|
|
|
Setting the original memory.limit_in_bytes below the current usage was
|
|
subject to a race condition, where concurrent charges could cause the
|
|
limit setting to fail. memory.max on the other hand will first set the
|
|
limit to prevent new charges, and then reclaim and OOM kill until the
|
|
new limit is met - or the task writing to memory.max is killed.
|
|
|
|
The combined memory+swap accounting and limiting is replaced by real
|
|
control over swap space.
|
|
|
|
The main argument for a combined memory+swap facility in the original
|
|
cgroup design was that global or parental pressure would always be
|
|
able to swap all anonymous memory of a child group, regardless of the
|
|
child's own (possibly untrusted) configuration. However, untrusted
|
|
groups can sabotage swapping by other means - such as referencing its
|
|
anonymous memory in a tight loop - and an admin can not assume full
|
|
swappability when overcommitting untrusted jobs.
|
|
|
|
For trusted jobs, on the other hand, a combined counter is not an
|
|
intuitive userspace interface, and it flies in the face of the idea
|
|
that cgroup controllers should account and limit specific physical
|
|
resources. Swap space is a resource like all others in the system,
|
|
and that's why unified hierarchy allows distributing it separately.
|