153 lines
7.2 KiB
ReStructuredText
153 lines
7.2 KiB
ReStructuredText
.. _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_threshold_percent``
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attribute, e. g.::
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echo 80 > /sys/module/zswap/parameters/accept_threshold_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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