1Overview:
2
3Zswap is a lightweight compressed cache for swap pages. It takes pages that are
4in the process of being swapped out and attempts to compress them into a
5dynamically allocated RAM-based memory pool.  zswap basically trades CPU cycles
6for potentially reduced swap I/O.�� This trade-off can also result in a
7significant performance improvement if reads from the compressed cache are
8faster than reads from a swap device.
9
10NOTE: Zswap is a new feature as of v3.11 and interacts heavily with memory
11reclaim.  This interaction has not been fully explored on the large set of
12potential configurations and workloads that exist.  For this reason, zswap
13is a work in progress and should be considered experimental.
14
15Some potential benefits:
16* Desktop/laptop users with limited RAM capacities can mitigate the
17������ performance impact of swapping.
18* Overcommitted guests that share a common I/O resource can
19������ dramatically reduce their swap I/O pressure, avoiding heavy handed I/O
20    throttling by the hypervisor.��This allows more work to get done with less
21    impact to the guest workload and guests sharing the I/O subsystem
22* Users with SSDs as swap devices can extend the life of the device by
23������ drastically reducing life-shortening writes.
24
25Zswap evicts pages from compressed cache on an LRU basis to the backing swap
26device when the compressed pool reaches its size limit.  This requirement had
27been identified in prior community discussions.
28
29Zswap is disabled by default but can be enabled at boot time by setting
30the "enabled" attribute to 1 at boot time. ie: zswap.enabled=1.  Zswap
31can also be enabled and disabled at runtime using the sysfs interface.
32An example command to enable zswap at runtime, assuming sysfs is mounted
33at /sys, is:
34
35echo 1 > /sys/module/zswap/parameters/enabled
36
37When zswap is disabled at runtime it will stop storing pages that are
38being swapped out.  However, it will _not_ immediately write out or fault
39back into memory all of the pages stored in the compressed pool.  The
40pages stored in zswap will remain in the compressed pool until they are
41either invalidated or faulted back into memory.  In order to force all
42pages out of the compressed pool, a swapoff on the swap device(s) will
43fault back into memory all swapped out pages, including those in the
44compressed pool.
45
46Design:
47
48Zswap receives pages for compression through the Frontswap API and is able to
49evict pages from its own compressed pool on an LRU basis and write them back to
50the backing swap device in the case that the compressed pool is full.
51
52Zswap makes use of zpool for the managing the compressed memory pool.  Each
53allocation in zpool is not directly accessible by address.  Rather, a handle is
54returned by the allocation routine and that handle must be mapped before being
55accessed.  The compressed memory pool grows on demand and shrinks as compressed
56pages are freed.  The pool is not preallocated.  By default, a zpool of type
57zbud is created, but it can be selected at boot time by setting the "zpool"
58attribute, e.g. zswap.zpool=zbud.  It can also be changed at runtime using the
59sysfs "zpool" attribute, e.g.
60
61echo zbud > /sys/module/zswap/parameters/zpool
62
63The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which
64means the compression ratio will always be 2:1 or worse (because of half-full
65zbud pages).  The zsmalloc type zpool has a more complex compressed page
66storage method, and it can achieve greater storage densities.  However,
67zsmalloc does not implement compressed page eviction, so once zswap fills it
68cannot evict the oldest page, it can only reject new pages.
69
70When a swap page is passed from frontswap to zswap, zswap maintains a mapping
71of the swap entry, a combination of the swap type and swap offset, to the zpool
72handle that references that compressed swap page.  This mapping is achieved
73with a red-black tree per swap type.  The swap offset is the search key for the
74tree nodes.
75
76During a page fault on a PTE that is a swap entry, frontswap calls the zswap
77load function to decompress the page into the page allocated by the page fault
78handler.
79
80Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
81in the swap_map goes to 0) the swap code calls the zswap invalidate function,
82via frontswap, to free the compressed entry.
83
84Zswap seeks to be simple in its policies.  Sysfs attributes allow for one user
85controlled policy:
86* max_pool_percent - The maximum percentage of memory that the compressed
87    pool can occupy.
88
89The default compressor is lzo, but it can be selected at boot time by setting
90the ���compressor��� attribute, e.g. zswap.compressor=lzo.  It can also be changed
91at runtime using the sysfs "compressor" attribute, e.g.
92
93echo lzo > /sys/module/zswap/parameters/compressor
94
95When the zpool and/or compressor parameter is changed at runtime, any existing
96compressed pages are not modified; they are left in their own zpool.  When a
97request is made for a page in an old zpool, it is uncompressed using its
98original compressor.  Once all pages are removed from an old zpool, the zpool
99and its compressor are freed.
100
101A debugfs interface is provided for various statistic about pool size, number
102of pages stored, and various counters for the reasons pages are rejected.
103