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Documentation / vm / zswap.txt




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Based on kernel version 3.16. Page generated on 2014-08-06 21:41 EST.

1	Overview:
2	
3	Zswap is a lightweight compressed cache for swap pages. It takes pages that are
4	in the process of being swapped out and attempts to compress them into a
5	dynamically allocated RAM-based memory pool.  zswap basically trades CPU cycles
6	for potentially reduced swap I/O.  This trade-off can also result in a
7	significant performance improvement if reads from the compressed cache are
8	faster than reads from a swap device.
9	
10	NOTE: Zswap is a new feature as of v3.11 and interacts heavily with memory
11	reclaim.  This interaction has not been fully explored on the large set of
12	potential configurations and workloads that exist.  For this reason, zswap
13	is a work in progress and should be considered experimental.
14	
15	Some 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	
25	Zswap evicts pages from compressed cache on an LRU basis to the backing swap
26	device when the compressed pool reaches its size limit.  This requirement had
27	been identified in prior community discussions.
28	
29	To enabled zswap, the "enabled" attribute must be set to 1 at boot time.  e.g.
30	zswap.enabled=1
31	
32	Design:
33	
34	Zswap receives pages for compression through the Frontswap API and is able to
35	evict pages from its own compressed pool on an LRU basis and write them back to
36	the backing swap device in the case that the compressed pool is full.
37	
38	Zswap makes use of zbud for the managing the compressed memory pool.  Each
39	allocation in zbud is not directly accessible by address.  Rather, a handle is
40	returned by the allocation routine and that handle must be mapped before being
41	accessed.  The compressed memory pool grows on demand and shrinks as compressed
42	pages are freed.  The pool is not preallocated.
43	
44	When a swap page is passed from frontswap to zswap, zswap maintains a mapping
45	of the swap entry, a combination of the swap type and swap offset, to the zbud
46	handle that references that compressed swap page.  This mapping is achieved
47	with a red-black tree per swap type.  The swap offset is the search key for the
48	tree nodes.
49	
50	During a page fault on a PTE that is a swap entry, frontswap calls the zswap
51	load function to decompress the page into the page allocated by the page fault
52	handler.
53	
54	Once there are no PTEs referencing a swap page stored in zswap (i.e. the count
55	in the swap_map goes to 0) the swap code calls the zswap invalidate function,
56	via frontswap, to free the compressed entry.
57	
58	Zswap seeks to be simple in its policies.  Sysfs attributes allow for one user
59	controlled policy:
60	* max_pool_percent - The maximum percentage of memory that the compressed
61	    pool can occupy.
62	
63	Zswap allows the compressor to be selected at kernel boot time by setting the
64	“compressor” attribute.  The default compressor is lzo.  e.g.
65	zswap.compressor=deflate
66	
67	A debugfs interface is provided for various statistic about pool size, number
68	of pages stored, and various counters for the reasons pages are rejected.
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