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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. However, the
- existing pages which are marked as same-value filled pages remain stored
- unchanged in zswap until they are either loaded or invalidated.
- 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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