memcg: update documentation
Some information are old, and I think current document doesn't work as "a guide for users". We need summary of all of our controls, at least. Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> Reviewed-by: Randy Dunlap <randy.dunlap@oracle.com> Cc: Balbir Singh <balbir@in.ibm.com> Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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@ -1,18 +1,15 @@
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Memory Resource Controller
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NOTE: The Memory Resource Controller has been generically been referred
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to as the memory controller in this document. Do not confuse memory controller
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used here with the memory controller that is used in hardware.
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to as the memory controller in this document. Do not confuse memory
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controller used here with the memory controller that is used in hardware.
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Salient features
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a. Enable control of Anonymous, Page Cache (mapped and unmapped) and
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Swap Cache memory pages.
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b. The infrastructure allows easy addition of other types of memory to control
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c. Provides *zero overhead* for non memory controller users
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d. Provides a double LRU: global memory pressure causes reclaim from the
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global LRU; a cgroup on hitting a limit, reclaims from the per
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cgroup LRU
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(For editors)
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In this document:
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When we mention a cgroup (cgroupfs's directory) with memory controller,
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we call it "memory cgroup". When you see git-log and source code, you'll
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see patch's title and function names tend to use "memcg".
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In this document, we avoid using it.
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Benefits and Purpose of the memory controller
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@ -33,6 +30,45 @@ d. A CD/DVD burner could control the amount of memory used by the
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e. There are several other use cases, find one or use the controller just
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for fun (to learn and hack on the VM subsystem).
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Current Status: linux-2.6.34-mmotm(development version of 2010/April)
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Features:
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- accounting anonymous pages, file caches, swap caches usage and limiting them.
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- private LRU and reclaim routine. (system's global LRU and private LRU
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work independently from each other)
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- optionally, memory+swap usage can be accounted and limited.
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- hierarchical accounting
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- soft limit
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- moving(recharging) account at moving a task is selectable.
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- usage threshold notifier
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- oom-killer disable knob and oom-notifier
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- Root cgroup has no limit controls.
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Kernel memory and Hugepages are not under control yet. We just manage
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pages on LRU. To add more controls, we have to take care of performance.
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Brief summary of control files.
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tasks # attach a task(thread) and show list of threads
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cgroup.procs # show list of processes
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cgroup.event_control # an interface for event_fd()
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memory.usage_in_bytes # show current memory(RSS+Cache) usage.
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memory.memsw.usage_in_bytes # show current memory+Swap usage
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memory.limit_in_bytes # set/show limit of memory usage
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memory.memsw.limit_in_bytes # set/show limit of memory+Swap usage
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memory.failcnt # show the number of memory usage hits limits
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memory.memsw.failcnt # show the number of memory+Swap hits limits
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memory.max_usage_in_bytes # show max memory usage recorded
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memory.memsw.usage_in_bytes # show max memory+Swap usage recorded
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memory.soft_limit_in_bytes # set/show soft limit of memory usage
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memory.stat # show various statistics
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memory.use_hierarchy # set/show hierarchical account enabled
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memory.force_empty # trigger forced move charge to parent
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memory.swappiness # set/show swappiness parameter of vmscan
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(See sysctl's vm.swappiness)
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memory.move_charge_at_immigrate # set/show controls of moving charges
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memory.oom_control # set/show oom controls.
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1. History
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The memory controller has a long history. A request for comments for the memory
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@ -106,14 +142,14 @@ the necessary data structures and check if the cgroup that is being charged
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is over its limit. If it is then reclaim is invoked on the cgroup.
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More details can be found in the reclaim section of this document.
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If everything goes well, a page meta-data-structure called page_cgroup is
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allocated and associated with the page. This routine also adds the page to
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the per cgroup LRU.
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updated. page_cgroup has its own LRU on cgroup.
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(*) page_cgroup structure is allocated at boot/memory-hotplug time.
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2.2.1 Accounting details
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All mapped anon pages (RSS) and cache pages (Page Cache) are accounted.
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(some pages which never be reclaimable and will not be on global LRU
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are not accounted. we just accounts pages under usual vm management.)
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Some pages which are never reclaimable and will not be on the global LRU
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are not accounted. We just account pages under usual VM management.
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RSS pages are accounted at page_fault unless they've already been accounted
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for earlier. A file page will be accounted for as Page Cache when it's
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@ -121,12 +157,19 @@ inserted into inode (radix-tree). While it's mapped into the page tables of
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processes, duplicate accounting is carefully avoided.
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A RSS page is unaccounted when it's fully unmapped. A PageCache page is
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unaccounted when it's removed from radix-tree.
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unaccounted when it's removed from radix-tree. Even if RSS pages are fully
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unmapped (by kswapd), they may exist as SwapCache in the system until they
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are really freed. Such SwapCaches also also accounted.
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A swapped-in page is not accounted until it's mapped.
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Note: The kernel does swapin-readahead and read multiple swaps at once.
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This means swapped-in pages may contain pages for other tasks than a task
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causing page fault. So, we avoid accounting at swap-in I/O.
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At page migration, accounting information is kept.
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Note: we just account pages-on-lru because our purpose is to control amount
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of used pages. not-on-lru pages are tend to be out-of-control from vm view.
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Note: we just account pages-on-LRU because our purpose is to control amount
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of used pages; not-on-LRU pages tend to be out-of-control from VM view.
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2.3 Shared Page Accounting
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@ -143,6 +186,7 @@ caller of swapoff rather than the users of shmem.
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2.4 Swap Extension (CONFIG_CGROUP_MEM_RES_CTLR_SWAP)
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Swap Extension allows you to record charge for swap. A swapped-in page is
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charged back to original page allocator if possible.
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@ -150,13 +194,20 @@ When swap is accounted, following files are added.
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- memory.memsw.usage_in_bytes.
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- memory.memsw.limit_in_bytes.
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usage of mem+swap is limited by memsw.limit_in_bytes.
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memsw means memory+swap. Usage of memory+swap is limited by
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memsw.limit_in_bytes.
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* why 'mem+swap' rather than swap.
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Example: Assume a system with 4G of swap. A task which allocates 6G of memory
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(by mistake) under 2G memory limitation will use all swap.
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In this case, setting memsw.limit_in_bytes=3G will prevent bad use of swap.
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By using memsw limit, you can avoid system OOM which can be caused by swap
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shortage.
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* why 'memory+swap' rather than swap.
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The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
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to move account from memory to swap...there is no change in usage of
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mem+swap. In other words, when we want to limit the usage of swap without
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affecting global LRU, mem+swap limit is better than just limiting swap from
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memory+swap. In other words, when we want to limit the usage of swap without
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affecting global LRU, memory+swap limit is better than just limiting swap from
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OS point of view.
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* What happens when a cgroup hits memory.memsw.limit_in_bytes
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@ -168,12 +219,12 @@ it by cgroup.
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2.5 Reclaim
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Each cgroup maintains a per cgroup LRU that consists of an active
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and inactive list. When a cgroup goes over its limit, we first try
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Each cgroup maintains a per cgroup LRU which has the same structure as
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global VM. When a cgroup goes over its limit, we first try
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to reclaim memory from the cgroup so as to make space for the new
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pages that the cgroup has touched. If the reclaim is unsuccessful,
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an OOM routine is invoked to select and kill the bulkiest task in the
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cgroup.
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cgroup. (See 10. OOM Control below.)
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The reclaim algorithm has not been modified for cgroups, except that
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pages that are selected for reclaiming come from the per cgroup LRU
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When oom event notifier is registered, event will be delivered.
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(See oom_control section)
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2. Locking
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2.6 Locking
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The memory controller uses the following hierarchy
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lock_page_cgroup()/unlock_page_cgroup() should not be called under
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mapping->tree_lock.
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1. zone->lru_lock is used for selecting pages to be isolated
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2. mem->per_zone->lru_lock protects the per cgroup LRU (per zone)
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3. lock_page_cgroup() is used to protect page->page_cgroup
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Other lock order is following:
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PG_locked.
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mm->page_table_lock
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zone->lru_lock
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lock_page_cgroup.
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In many cases, just lock_page_cgroup() is called.
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per-zone-per-cgroup LRU (cgroup's private LRU) is just guarded by
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zone->lru_lock, it has no lock of its own.
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3. User Interface
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a. Enable CONFIG_CGROUPS
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b. Enable CONFIG_RESOURCE_COUNTERS
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c. Enable CONFIG_CGROUP_MEM_RES_CTLR
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d. Enable CONFIG_CGROUP_MEM_RES_CTLR_SWAP (to use swap extension)
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1. Prepare the cgroups
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# mkdir -p /cgroups
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2. Make the new group and move bash into it
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# mkdir /cgroups/0
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# echo $$ > /cgroups/0/tasks
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# echo $$ > /cgroups/0/tasks
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Since now we're in the 0 cgroup,
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We can alter the memory limit:
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Since now we're in the 0 cgroup, we can alter the memory limit:
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# echo 4M > /cgroups/0/memory.limit_in_bytes
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NOTE: We can use a suffix (k, K, m, M, g or G) to indicate values in kilo,
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mega or gigabytes.
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mega or gigabytes. (Here, Kilo, Mega, Giga are Kibibytes, Mebibytes, Gibibytes.)
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NOTE: We can write "-1" to reset the *.limit_in_bytes(unlimited).
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NOTE: We cannot set limits on the root cgroup any more.
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# cat /cgroups/0/memory.limit_in_bytes
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4194304
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NOTE: The interface has now changed to display the usage in bytes
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instead of pages
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We can check the usage:
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# cat /cgroups/0/memory.usage_in_bytes
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1216512
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A successful write to this file does not guarantee a successful set of
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this limit to the value written into the file. This can be due to a
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this limit to the value written into the file. This can be due to a
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number of factors, such as rounding up to page boundaries or the total
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availability of memory on the system. The user is required to re-read
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availability of memory on the system. The user is required to re-read
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this file after a write to guarantee the value committed by the kernel.
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# echo 1 > memory.limit_in_bytes
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@ -248,15 +303,23 @@ caches, RSS and Active pages/Inactive pages are shown.
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4. Testing
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Balbir posted lmbench, AIM9, LTP and vmmstress results [10] and [11].
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Apart from that v6 has been tested with several applications and regular
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daily use. The controller has also been tested on the PPC64, x86_64 and
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UML platforms.
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For testing features and implementation, see memcg_test.txt.
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Performance test is also important. To see pure memory controller's overhead,
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testing on tmpfs will give you good numbers of small overheads.
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Example: do kernel make on tmpfs.
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Page-fault scalability is also important. At measuring parallel
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page fault test, multi-process test may be better than multi-thread
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test because it has noise of shared objects/status.
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But the above two are testing extreme situations.
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Trying usual test under memory controller is always helpful.
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4.1 Troubleshooting
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Sometimes a user might find that the application under a cgroup is
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terminated. There are several causes for this:
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terminated by OOM killer. There are several causes for this:
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1. The cgroup limit is too low (just too low to do anything useful)
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2. The user is using anonymous memory and swap is turned off or too low
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A sync followed by echo 1 > /proc/sys/vm/drop_caches will help get rid of
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some of the pages cached in the cgroup (page cache pages).
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To know what happens, disable OOM_Kill by 10. OOM Control(see below) and
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seeing what happens will be helpful.
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4.2 Task migration
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When a task migrates from one cgroup to another, its charge is not
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remain charged to it, the charge is dropped when the page is freed or
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reclaimed.
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Note: You can move charges of a task along with task migration. See 8.
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You can move charges of a task along with task migration.
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See 8. "Move charges at task migration"
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4.3 Removing a cgroup
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A cgroup can be removed by rmdir, but as discussed in sections 4.1 and 4.2, a
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cgroup might have some charge associated with it, even though all
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tasks have migrated away from it.
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Such charges are freed(at default) or moved to its parent. When moved,
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both of RSS and CACHES are moved to parent.
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If both of them are busy, rmdir() returns -EBUSY. See 5.1 Also.
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tasks have migrated away from it. (because we charge against pages, not
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against tasks.)
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Such charges are freed or moved to their parent. At moving, both of RSS
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and CACHES are moved to parent.
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rmdir() may return -EBUSY if freeing/moving fails. See 5.1 also.
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Charges recorded in swap information is not updated at removal of cgroup.
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Recorded information is discarded and a cgroup which uses swap (swapcache)
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@ -296,10 +365,10 @@ will be charged as a new owner of it.
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# echo 0 > memory.force_empty
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Almost all pages tracked by this memcg will be unmapped and freed. Some of
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pages cannot be freed because it's locked or in-use. Such pages are moved
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to parent and this cgroup will be empty. But this may return -EBUSY in
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some too busy case.
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Almost all pages tracked by this memory cgroup will be unmapped and freed.
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Some pages cannot be freed because they are locked or in-use. Such pages are
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moved to parent and this cgroup will be empty. This may return -EBUSY if
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VM is too busy to free/move all pages immediately.
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Typical use case of this interface is that calling this before rmdir().
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Because rmdir() moves all pages to parent, some out-of-use page caches can be
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memory.stat file includes following statistics
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# per-memory cgroup local status
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cache - # of bytes of page cache memory.
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rss - # of bytes of anonymous and swap cache memory.
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mapped_file - # of bytes of mapped file (includes tmpfs/shmem)
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pgpgin - # of pages paged in (equivalent to # of charging events).
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pgpgout - # of pages paged out (equivalent to # of uncharging events).
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active_anon - # of bytes of anonymous and swap cache memory on active
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lru list.
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swap - # of bytes of swap usage
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inactive_anon - # of bytes of anonymous memory and swap cache memory on
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inactive lru list.
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active_file - # of bytes of file-backed memory on active lru list.
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inactive_file - # of bytes of file-backed memory on inactive lru list.
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LRU list.
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active_anon - # of bytes of anonymous and swap cache memory on active
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inactive LRU list.
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inactive_file - # of bytes of file-backed memory on inactive LRU list.
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active_file - # of bytes of file-backed memory on active LRU list.
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unevictable - # of bytes of memory that cannot be reclaimed (mlocked etc).
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The following additional stats are dependent on CONFIG_DEBUG_VM.
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# status considering hierarchy (see memory.use_hierarchy settings)
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hierarchical_memory_limit - # of bytes of memory limit with regard to hierarchy
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under which the memory cgroup is
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hierarchical_memsw_limit - # of bytes of memory+swap limit with regard to
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hierarchy under which memory cgroup is.
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total_cache - sum of all children's "cache"
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total_rss - sum of all children's "rss"
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total_mapped_file - sum of all children's "cache"
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total_pgpgin - sum of all children's "pgpgin"
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total_pgpgout - sum of all children's "pgpgout"
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total_swap - sum of all children's "swap"
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total_inactive_anon - sum of all children's "inactive_anon"
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total_active_anon - sum of all children's "active_anon"
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total_inactive_file - sum of all children's "inactive_file"
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total_active_file - sum of all children's "active_file"
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total_unevictable - sum of all children's "unevictable"
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# The following additional stats are dependent on CONFIG_DEBUG_VM.
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inactive_ratio - VM internal parameter. (see mm/page_alloc.c)
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recent_rotated_anon - VM internal parameter. (see mm/vmscan.c)
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@ -330,24 +421,37 @@ recent_scanned_anon - VM internal parameter. (see mm/vmscan.c)
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recent_scanned_file - VM internal parameter. (see mm/vmscan.c)
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Memo:
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recent_rotated means recent frequency of lru rotation.
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recent_scanned means recent # of scans to lru.
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recent_rotated means recent frequency of LRU rotation.
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recent_scanned means recent # of scans to LRU.
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showing for better debug please see the code for meanings.
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Note:
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Only anonymous and swap cache memory is listed as part of 'rss' stat.
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This should not be confused with the true 'resident set size' or the
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amount of physical memory used by the cgroup. Per-cgroup rss
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accounting is not done yet.
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amount of physical memory used by the cgroup.
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'rss + file_mapped" will give you resident set size of cgroup.
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(Note: file and shmem may be shared among other cgroups. In that case,
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file_mapped is accounted only when the memory cgroup is owner of page
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cache.)
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5.3 swappiness
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Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
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Following cgroups' swappiness can't be changed.
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- root cgroup (uses /proc/sys/vm/swappiness).
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- a cgroup which uses hierarchy and it has child cgroup.
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- a cgroup which uses hierarchy and not the root of hierarchy.
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Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
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Following cgroups' swappiness can't be changed.
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- root cgroup (uses /proc/sys/vm/swappiness).
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- a cgroup which uses hierarchy and it has other cgroup(s) below it.
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- a cgroup which uses hierarchy and not the root of hierarchy.
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5.4 failcnt
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A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
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This failcnt(== failure count) shows the number of times that a usage counter
|
||||
hit its limit. When a memory cgroup hits a limit, failcnt increases and
|
||||
memory under it will be reclaimed.
|
||||
|
||||
You can reset failcnt by writing 0 to failcnt file.
|
||||
# echo 0 > .../memory.failcnt
|
||||
|
||||
6. Hierarchy support
|
||||
|
||||
|
@ -366,13 +470,13 @@ hierarchy
|
|||
|
||||
In the diagram above, with hierarchical accounting enabled, all memory
|
||||
usage of e, is accounted to its ancestors up until the root (i.e, c and root),
|
||||
that has memory.use_hierarchy enabled. If one of the ancestors goes over its
|
||||
that has memory.use_hierarchy enabled. If one of the ancestors goes over its
|
||||
limit, the reclaim algorithm reclaims from the tasks in the ancestor and the
|
||||
children of the ancestor.
|
||||
|
||||
6.1 Enabling hierarchical accounting and reclaim
|
||||
|
||||
The memory controller by default disables the hierarchy feature. Support
|
||||
A memory cgroup by default disables the hierarchy feature. Support
|
||||
can be enabled by writing 1 to memory.use_hierarchy file of the root cgroup
|
||||
|
||||
# echo 1 > memory.use_hierarchy
|
||||
|
@ -382,10 +486,10 @@ The feature can be disabled by
|
|||
# echo 0 > memory.use_hierarchy
|
||||
|
||||
NOTE1: Enabling/disabling will fail if the cgroup already has other
|
||||
cgroups created below it.
|
||||
cgroups created below it.
|
||||
|
||||
NOTE2: When panic_on_oom is set to "2", the whole system will panic in
|
||||
case of an oom event in any cgroup.
|
||||
case of an OOM event in any cgroup.
|
||||
|
||||
7. Soft limits
|
||||
|
||||
|
@ -395,7 +499,7 @@ is to allow control groups to use as much of the memory as needed, provided
|
|||
a. There is no memory contention
|
||||
b. They do not exceed their hard limit
|
||||
|
||||
When the system detects memory contention or low memory control groups
|
||||
When the system detects memory contention or low memory, control groups
|
||||
are pushed back to their soft limits. If the soft limit of each control
|
||||
group is very high, they are pushed back as much as possible to make
|
||||
sure that one control group does not starve the others of memory.
|
||||
|
@ -409,7 +513,7 @@ it gets invoked from balance_pgdat (kswapd).
|
|||
7.1 Interface
|
||||
|
||||
Soft limits can be setup by using the following commands (in this example we
|
||||
assume a soft limit of 256 megabytes)
|
||||
assume a soft limit of 256 MiB)
|
||||
|
||||
# echo 256M > memory.soft_limit_in_bytes
|
||||
|
||||
|
@ -445,7 +549,7 @@ Note: Charges are moved only when you move mm->owner, IOW, a leader of a thread
|
|||
Note: If we cannot find enough space for the task in the destination cgroup, we
|
||||
try to make space by reclaiming memory. Task migration may fail if we
|
||||
cannot make enough space.
|
||||
Note: It can take several seconds if you move charges in giga bytes order.
|
||||
Note: It can take several seconds if you move charges much.
|
||||
|
||||
And if you want disable it again:
|
||||
|
||||
|
@ -465,7 +569,7 @@ memory cgroup.
|
|||
| enable Swap Extension(see 2.4) to enable move of swap charges.
|
||||
-----+------------------------------------------------------------------------
|
||||
1 | A charge of file pages(normal file, tmpfs file(e.g. ipc shared memory)
|
||||
| and swaps of tmpfs file) mmaped by the target task. Unlike the case of
|
||||
| and swaps of tmpfs file) mmapped by the target task. Unlike the case of
|
||||
| anonymous pages, file pages(and swaps) in the range mmapped by the task
|
||||
| will be moved even if the task hasn't done page fault, i.e. they might
|
||||
| not be the task's "RSS", but other task's "RSS" that maps the same file.
|
||||
|
@ -482,15 +586,15 @@ memory cgroup.
|
|||
|
||||
9. Memory thresholds
|
||||
|
||||
Memory controler implements memory thresholds using cgroups notification
|
||||
Memory cgroup implements memory thresholds using cgroups notification
|
||||
API (see cgroups.txt). It allows to register multiple memory and memsw
|
||||
thresholds and gets notifications when it crosses.
|
||||
|
||||
To register a threshold application need:
|
||||
- create an eventfd using eventfd(2);
|
||||
- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
|
||||
- write string like "<event_fd> <memory.usage_in_bytes> <threshold>" to
|
||||
cgroup.event_control.
|
||||
- create an eventfd using eventfd(2);
|
||||
- open memory.usage_in_bytes or memory.memsw.usage_in_bytes;
|
||||
- write string like "<event_fd> <fd of memory.usage_in_bytes> <threshold>" to
|
||||
cgroup.event_control.
|
||||
|
||||
Application will be notified through eventfd when memory usage crosses
|
||||
threshold in any direction.
|
||||
|
@ -501,27 +605,28 @@ It's applicable for root and non-root cgroup.
|
|||
|
||||
memory.oom_control file is for OOM notification and other controls.
|
||||
|
||||
Memory controler implements oom notifier using cgroup notification
|
||||
API (See cgroups.txt). It allows to register multiple oom notification
|
||||
delivery and gets notification when oom happens.
|
||||
Memory cgroup implements OOM notifier using cgroup notification
|
||||
API (See cgroups.txt). It allows to register multiple OOM notification
|
||||
delivery and gets notification when OOM happens.
|
||||
|
||||
To register a notifier, application need:
|
||||
- create an eventfd using eventfd(2)
|
||||
- open memory.oom_control file
|
||||
- write string like "<event_fd> <memory.oom_control>" to cgroup.event_control
|
||||
- write string like "<event_fd> <fd of memory.oom_control>" to
|
||||
cgroup.event_control
|
||||
|
||||
Application will be notifier through eventfd when oom happens.
|
||||
Application will be notified through eventfd when OOM happens.
|
||||
OOM notification doesn't work for root cgroup.
|
||||
|
||||
You can disable oom-killer by writing "1" to memory.oom_control file.
|
||||
As.
|
||||
You can disable OOM-killer by writing "1" to memory.oom_control file, as:
|
||||
|
||||
#echo 1 > memory.oom_control
|
||||
|
||||
This operation is only allowed to the top cgroup of subhierarchy.
|
||||
If oom-killer is disabled, tasks under cgroup will hang/sleep
|
||||
in memcg's oom-waitq when they request accountable memory.
|
||||
This operation is only allowed to the top cgroup of sub-hierarchy.
|
||||
If OOM-killer is disabled, tasks under cgroup will hang/sleep
|
||||
in memory cgroup's OOM-waitqueue when they request accountable memory.
|
||||
|
||||
For running them, you have to relax the memcg's oom sitaution by
|
||||
For running them, you have to relax the memory cgroup's OOM status by
|
||||
* enlarge limit or reduce usage.
|
||||
To reduce usage,
|
||||
* kill some tasks.
|
||||
|
@ -532,7 +637,7 @@ Then, stopped tasks will work again.
|
|||
|
||||
At reading, current status of OOM is shown.
|
||||
oom_kill_disable 0 or 1 (if 1, oom-killer is disabled)
|
||||
under_oom 0 or 1 (if 1, the memcg is under OOM,tasks may
|
||||
under_oom 0 or 1 (if 1, the memory cgroup is under OOM, tasks may
|
||||
be stopped.)
|
||||
|
||||
11. TODO
|
||||
|
|
Loading…
Reference in a new issue