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Seth Jennings61b0d762013-07-10 16:05:05 -07001Overview:
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
Christian Hesse0151e3d2013-11-12 15:07:34 -080011reclaim. This interaction has not been fully explored on the large set of
Seth Jennings61b0d762013-07-10 16:05:05 -070012potential 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
Christian Hesse0151e3d2013-11-12 15:07:34 -080026device when the compressed pool reaches its size limit. This requirement had
Seth Jennings61b0d762013-07-10 16:05:05 -070027been identified in prior community discussions.
28
Dan Streetmanc00ed162015-06-25 15:00:35 -070029Zswap 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/modules/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.
Seth Jennings61b0d762013-07-10 16:05:05 -070045
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 zbud for the managing the compressed memory pool. Each
53allocation in zbud is not directly accessible by address. Rather, a handle is
Christian Hesse0151e3d2013-11-12 15:07:34 -080054returned by the allocation routine and that handle must be mapped before being
Seth Jennings61b0d762013-07-10 16:05:05 -070055accessed. The compressed memory pool grows on demand and shrinks as compressed
56pages are freed. The pool is not preallocated.
57
58When a swap page is passed from frontswap to zswap, zswap maintains a mapping
59of the swap entry, a combination of the swap type and swap offset, to the zbud
60handle that references that compressed swap page. This mapping is achieved
61with a red-black tree per swap type. The swap offset is the search key for the
62tree nodes.
63
64During a page fault on a PTE that is a swap entry, frontswap calls the zswap
65load function to decompress the page into the page allocated by the page fault
66handler.
67
68Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
69in the swap_map goes to 0) the swap code calls the zswap invalidate function,
70via frontswap, to free the compressed entry.
71
72Zswap seeks to be simple in its policies. Sysfs attributes allow for one user
Christian Hesse0151e3d2013-11-12 15:07:34 -080073controlled policy:
Seth Jennings61b0d762013-07-10 16:05:05 -070074* max_pool_percent - The maximum percentage of memory that the compressed
75 pool can occupy.
76
77Zswap allows the compressor to be selected at kernel boot time by setting the
78“compressor” attribute. The default compressor is lzo. e.g.
79zswap.compressor=deflate
80
81A debugfs interface is provided for various statistic about pool size, number
82of pages stored, and various counters for the reasons pages are rejected.