![Maciej S. Szmigiero](/assets/img/avatar_default.png)
The compressed cache for swap pages (zswap) currently needs from 1 to 3 extra kernel command line parameters in order to make it work: it has to be enabled by adding a "zswap.enabled=1" command line parameter and if one wants a different compressor or pool allocator than the default lzo / zbud combination then these choices also need to be specified on the kernel command line in additional parameters. Using a different compressor and allocator for zswap is actually pretty common as guides often recommend using the lz4 / z3fold pair instead of the default one. In such case it is also necessary to remember to enable the appropriate compression algorithm and pool allocator in the kernel config manually. Let's avoid the need for adding these kernel command line parameters and automatically pull in the dependencies for the selected compressor algorithm and pool allocator by adding an appropriate default switches to Kconfig. The default values for these options match what the code was using previously as its defaults. Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Reviewed-by: Vitaly Wool <vitaly.wool@konsulko.com> Link: http://lkml.kernel.org/r/20200202000112.456103-1-mail@maciej.szmigiero.name Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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7.2 KiB
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153 lines
7.2 KiB
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.. _zswap:
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=====
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zswap
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=====
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Overview
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========
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Zswap is a lightweight compressed cache for swap pages. It takes pages that are
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in the process of being swapped out and attempts to compress them into a
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dynamically allocated RAM-based memory pool. zswap basically trades CPU cycles
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for potentially reduced swap I/O. This trade-off can also result in a
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significant performance improvement if reads from the compressed cache are
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faster than reads from a swap device.
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.. note::
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Zswap is a new feature as of v3.11 and interacts heavily with memory
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reclaim. This interaction has not been fully explored on the large set of
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potential configurations and workloads that exist. For this reason, zswap
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is a work in progress and should be considered experimental.
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Some potential benefits:
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* Desktop/laptop users with limited RAM capacities can mitigate the
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performance impact of swapping.
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* Overcommitted guests that share a common I/O resource can
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dramatically reduce their swap I/O pressure, avoiding heavy handed I/O
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throttling by the hypervisor. This allows more work to get done with less
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impact to the guest workload and guests sharing the I/O subsystem
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* Users with SSDs as swap devices can extend the life of the device by
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drastically reducing life-shortening writes.
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Zswap evicts pages from compressed cache on an LRU basis to the backing swap
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device when the compressed pool reaches its size limit. This requirement had
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been identified in prior community discussions.
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Whether Zswap is enabled at the boot time depends on whether
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the ``CONFIG_ZSWAP_DEFAULT_ON`` Kconfig option is enabled or not.
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This setting can then be overridden by providing the kernel command line
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``zswap.enabled=`` option, for example ``zswap.enabled=0``.
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Zswap can also be enabled and disabled at runtime using the sysfs interface.
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An example command to enable zswap at runtime, assuming sysfs is mounted
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at ``/sys``, is::
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echo 1 > /sys/module/zswap/parameters/enabled
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When zswap is disabled at runtime it will stop storing pages that are
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being swapped out. However, it will _not_ immediately write out or fault
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back into memory all of the pages stored in the compressed pool. The
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pages stored in zswap will remain in the compressed pool until they are
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either invalidated or faulted back into memory. In order to force all
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pages out of the compressed pool, a swapoff on the swap device(s) will
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fault back into memory all swapped out pages, including those in the
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compressed pool.
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Design
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======
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Zswap receives pages for compression through the Frontswap API and is able to
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evict pages from its own compressed pool on an LRU basis and write them back to
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the backing swap device in the case that the compressed pool is full.
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Zswap makes use of zpool for the managing the compressed memory pool. Each
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allocation in zpool is not directly accessible by address. Rather, a handle is
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returned by the allocation routine and that handle must be mapped before being
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accessed. The compressed memory pool grows on demand and shrinks as compressed
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pages are freed. The pool is not preallocated. By default, a zpool
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of type selected in ``CONFIG_ZSWAP_ZPOOL_DEFAULT`` Kconfig option is created,
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but it can be overridden at boot time by setting the ``zpool`` attribute,
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e.g. ``zswap.zpool=zbud``. It can also be changed at runtime using the sysfs
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``zpool`` attribute, e.g.::
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echo zbud > /sys/module/zswap/parameters/zpool
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The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which
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means the compression ratio will always be 2:1 or worse (because of half-full
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zbud pages). The zsmalloc type zpool has a more complex compressed page
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storage method, and it can achieve greater storage densities. However,
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zsmalloc does not implement compressed page eviction, so once zswap fills it
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cannot evict the oldest page, it can only reject new pages.
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When a swap page is passed from frontswap to zswap, zswap maintains a mapping
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of the swap entry, a combination of the swap type and swap offset, to the zpool
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handle that references that compressed swap page. This mapping is achieved
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with a red-black tree per swap type. The swap offset is the search key for the
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tree nodes.
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During a page fault on a PTE that is a swap entry, frontswap calls the zswap
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load function to decompress the page into the page allocated by the page fault
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handler.
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Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
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in the swap_map goes to 0) the swap code calls the zswap invalidate function,
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via frontswap, to free the compressed entry.
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Zswap seeks to be simple in its policies. Sysfs attributes allow for one user
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controlled policy:
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* max_pool_percent - The maximum percentage of memory that the compressed
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pool can occupy.
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The default compressor is selected in ``CONFIG_ZSWAP_COMPRESSOR_DEFAULT``
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Kconfig option, but it can be overridden at boot time by setting the
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``compressor`` attribute, e.g. ``zswap.compressor=lzo``.
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It can also be changed at runtime using the sysfs "compressor"
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attribute, e.g.::
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echo lzo > /sys/module/zswap/parameters/compressor
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When the zpool and/or compressor parameter is changed at runtime, any existing
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compressed pages are not modified; they are left in their own zpool. When a
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request is made for a page in an old zpool, it is uncompressed using its
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original compressor. Once all pages are removed from an old zpool, the zpool
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and its compressor are freed.
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Some of the pages in zswap are same-value filled pages (i.e. contents of the
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page have same value or repetitive pattern). These pages include zero-filled
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pages and they are handled differently. During store operation, a page is
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checked if it is a same-value filled page before compressing it. If true, the
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compressed length of the page is set to zero and the pattern or same-filled
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value is stored.
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Same-value filled pages identification feature is enabled by default and can be
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disabled at boot time by setting the ``same_filled_pages_enabled`` attribute
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to 0, e.g. ``zswap.same_filled_pages_enabled=0``. It can also be enabled and
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disabled at runtime using the sysfs ``same_filled_pages_enabled``
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attribute, e.g.::
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echo 1 > /sys/module/zswap/parameters/same_filled_pages_enabled
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When zswap same-filled page identification is disabled at runtime, it will stop
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checking for the same-value filled pages during store operation. However, the
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existing pages which are marked as same-value filled pages remain stored
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unchanged in zswap until they are either loaded or invalidated.
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To prevent zswap from shrinking pool when zswap is full and there's a high
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pressure on swap (this will result in flipping pages in and out zswap pool
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without any real benefit but with a performance drop for the system), a
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special parameter has been introduced to implement a sort of hysteresis to
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refuse taking pages into zswap pool until it has sufficient space if the limit
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has been hit. To set the threshold at which zswap would start accepting pages
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again after it became full, use the sysfs ``accept_threhsold_percent``
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attribute, e. g.::
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echo 80 > /sys/module/zswap/parameters/accept_threhsold_percent
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Setting this parameter to 100 will disable the hysteresis.
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A debugfs interface is provided for various statistic about pool size, number
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of pages stored, same-value filled pages and various counters for the reasons
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pages are rejected.
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