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

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