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Based on kernel version 3.2. Page generated on 2012-01-05 23:29 EST.

1	How to use the Kernel Samepage Merging feature
2	----------------------------------------------
3	
4	KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
5	added to the Linux kernel in 2.6.32.  See mm/ksm.c for its implementation,
6	and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/
7	
8	The KSM daemon ksmd periodically scans those areas of user memory which
9	have been registered with it, looking for pages of identical content which
10	can be replaced by a single write-protected page (which is automatically
11	copied if a process later wants to update its content).
12	
13	KSM was originally developed for use with KVM (where it was known as
14	Kernel Shared Memory), to fit more virtual machines into physical memory,
15	by sharing the data common between them.  But it can be useful to any
16	application which generates many instances of the same data.
17	
18	KSM only merges anonymous (private) pages, never pagecache (file) pages.
19	KSM's merged pages were originally locked into kernel memory, but can now
20	be swapped out just like other user pages (but sharing is broken when they
21	are swapped back in: ksmd must rediscover their identity and merge again).
22	
23	KSM only operates on those areas of address space which an application
24	has advised to be likely candidates for merging, by using the madvise(2)
25	system call: int madvise(addr, length, MADV_MERGEABLE).
26	
27	The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel
28	that advice and restore unshared pages: whereupon KSM unmerges whatever
29	it merged in that range.  Note: this unmerging call may suddenly require
30	more memory than is available - possibly failing with EAGAIN, but more
31	probably arousing the Out-Of-Memory killer.
32	
33	If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
34	and MADV_UNMERGEABLE simply fail with EINVAL.  If the running kernel was
35	built with CONFIG_KSM=y, those calls will normally succeed: even if the
36	the KSM daemon is not currently running, MADV_MERGEABLE still registers
37	the range for whenever the KSM daemon is started; even if the range
38	cannot contain any pages which KSM could actually merge; even if
39	MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
40	
41	Like other madvise calls, they are intended for use on mapped areas of
42	the user address space: they will report ENOMEM if the specified range
43	includes unmapped gaps (though working on the intervening mapped areas),
44	and might fail with EAGAIN if not enough memory for internal structures.
45	
46	Applications should be considerate in their use of MADV_MERGEABLE,
47	restricting its use to areas likely to benefit.  KSM's scans may use a lot
48	of processing power: some installations will disable KSM for that reason.
49	
50	The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
51	readable by all but writable only by root:
52	
53	pages_to_scan    - how many present pages to scan before ksmd goes to sleep
54	                   e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan"
55	                   Default: 100 (chosen for demonstration purposes)
56	
57	sleep_millisecs  - how many milliseconds ksmd should sleep before next scan
58	                   e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs"
59	                   Default: 20 (chosen for demonstration purposes)
60	
61	run              - set 0 to stop ksmd from running but keep merged pages,
62	                   set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
63	                   set 2 to stop ksmd and unmerge all pages currently merged,
64	                         but leave mergeable areas registered for next run
65	                   Default: 0 (must be changed to 1 to activate KSM,
66	                               except if CONFIG_SYSFS is disabled)
67	
68	The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
69	
70	pages_shared     - how many shared pages are being used
71	pages_sharing    - how many more sites are sharing them i.e. how much saved
72	pages_unshared   - how many pages unique but repeatedly checked for merging
73	pages_volatile   - how many pages changing too fast to be placed in a tree
74	full_scans       - how many times all mergeable areas have been scanned
75	
76	A high ratio of pages_sharing to pages_shared indicates good sharing, but
77	a high ratio of pages_unshared to pages_sharing indicates wasted effort.
78	pages_volatile embraces several different kinds of activity, but a high
79	proportion there would also indicate poor use of madvise MADV_MERGEABLE.
80	
81	Izik Eidus,
82	Hugh Dickins, 17 Nov 2009
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