|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.
|Zswap is disabled by default but can be enabled at boot time by setting
|the "enabled" attribute to 1 at boot time. ie: zswap.enabled=1. 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
|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
|zbud is created, but it can be selected 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
|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
|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
|* max_pool_percent - The maximum percentage of memory that the compressed
| pool can occupy.
|The default compressor is lzo, but it can be selected 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.
|A debugfs interface is provided for various statistic about pool size, number
|of pages stored, and various counters for the reasons pages are rejected.