docs/vm: zswap.txt: convert to ReST format
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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Mike Rapoport
Jonathan Corbet
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Overview:
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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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@ -7,32 +14,34 @@ 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: 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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.. 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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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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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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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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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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Zswap is disabled by default but can be enabled at boot time by setting
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the "enabled" attribute to 1 at boot time. ie: zswap.enabled=1. Zswap
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the ``enabled`` attribute to 1 at boot time. ie: ``zswap.enabled=1``. Zswap
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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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at ``/sys``, is::
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echo 1 > /sys/module/zswap/parameters/enabled
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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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@ -43,7 +52,8 @@ 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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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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@ -53,12 +63,12 @@ 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 of type
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zbud is created, but it can be selected at boot time by setting the "zpool"
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attribute, e.g. zswap.zpool=zbud. It can also be changed at runtime using the
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sysfs "zpool" attribute, e.g.
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pages are freed. The pool is not preallocated. By default, a zpool
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of type zbud is created, but it can be selected at boot time by
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setting the ``zpool`` attribute, e.g. ``zswap.zpool=zbud``. It can
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also be changed at runtime using the sysfs ``zpool`` attribute, e.g.::
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echo zbud > /sys/module/zswap/parameters/zpool
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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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@ -83,14 +93,16 @@ 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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pool can occupy.
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The default compressor is lzo, but it can be selected at boot time by setting
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the “compressor” attribute, e.g. zswap.compressor=lzo. It can also be changed
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at runtime using the sysfs "compressor" attribute, e.g.
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The default compressor is lzo, but it can be selected at boot time by
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setting the ``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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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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@ -106,11 +118,12 @@ 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 to 0,
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e.g. zswap.same_filled_pages_enabled=0. It can also be enabled and disabled at
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runtime using the sysfs "same_filled_pages_enabled" attribute, e.g.
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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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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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