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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 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	If a region of memory must be split into at least one new MADV_MERGEABLE
42	or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
43	will exceed vm.max_map_count (see Documentation/sysctl/vm.txt).
44	
45	Like other madvise calls, they are intended for use on mapped areas of
46	the user address space: they will report ENOMEM if the specified range
47	includes unmapped gaps (though working on the intervening mapped areas),
48	and might fail with EAGAIN if not enough memory for internal structures.
49	
50	Applications should be considerate in their use of MADV_MERGEABLE,
51	restricting its use to areas likely to benefit.  KSM's scans may use a lot
52	of processing power: some installations will disable KSM for that reason.
53	
54	The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
55	readable by all but writable only by root:
56	
57	pages_to_scan    - how many present pages to scan before ksmd goes to sleep
58	                   e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan"
59	                   Default: 100 (chosen for demonstration purposes)
60	
61	sleep_millisecs  - how many milliseconds ksmd should sleep before next scan
62	                   e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs"
63	                   Default: 20 (chosen for demonstration purposes)
64	
65	merge_across_nodes - specifies if pages from different numa nodes can be merged.
66	                   When set to 0, ksm merges only pages which physically
67	                   reside in the memory area of same NUMA node. That brings
68	                   lower latency to access of shared pages. Systems with more
69	                   nodes, at significant NUMA distances, are likely to benefit
70	                   from the lower latency of setting 0. Smaller systems, which
71	                   need to minimize memory usage, are likely to benefit from
72	                   the greater sharing of setting 1 (default). You may wish to
73	                   compare how your system performs under each setting, before
74	                   deciding on which to use. merge_across_nodes setting can be
75	                   changed only when there are no ksm shared pages in system:
76	                   set run 2 to unmerge pages first, then to 1 after changing
77	                   merge_across_nodes, to remerge according to the new setting.
78	                   Default: 1 (merging across nodes as in earlier releases)
79	
80	run              - set 0 to stop ksmd from running but keep merged pages,
81	                   set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
82	                   set 2 to stop ksmd and unmerge all pages currently merged,
83	                         but leave mergeable areas registered for next run
84	                   Default: 0 (must be changed to 1 to activate KSM,
85	                               except if CONFIG_SYSFS is disabled)
86	
87	use_zero_pages   - specifies whether empty pages (i.e. allocated pages
88	                   that only contain zeroes) should be treated specially.
89	                   When set to 1, empty pages are merged with the kernel
90	                   zero page(s) instead of with each other as it would
91	                   happen normally. This can improve the performance on
92	                   architectures with coloured zero pages, depending on
93	                   the workload. Care should be taken when enabling this
94	                   setting, as it can potentially degrade the performance
95	                   of KSM for some workloads, for example if the checksums
96	                   of pages candidate for merging match the checksum of
97	                   an empty page. This setting can be changed at any time,
98	                   it is only effective for pages merged after the change.
99	                   Default: 0 (normal KSM behaviour as in earlier releases)
100	
101	max_page_sharing - Maximum sharing allowed for each KSM page. This
102	                   enforces a deduplication limit to avoid the virtual
103	                   memory rmap lists to grow too large. The minimum
104	                   value is 2 as a newly created KSM page will have at
105	                   least two sharers. The rmap walk has O(N)
106	                   complexity where N is the number of rmap_items
107	                   (i.e. virtual mappings) that are sharing the page,
108	                   which is in turn capped by max_page_sharing. So
109	                   this effectively spread the the linear O(N)
110	                   computational complexity from rmap walk context
111	                   over different KSM pages. The ksmd walk over the
112	                   stable_node "chains" is also O(N), but N is the
113	                   number of stable_node "dups", not the number of
114	                   rmap_items, so it has not a significant impact on
115	                   ksmd performance. In practice the best stable_node
116	                   "dup" candidate will be kept and found at the head
117	                   of the "dups" list. The higher this value the
118	                   faster KSM will merge the memory (because there
119	                   will be fewer stable_node dups queued into the
120	                   stable_node chain->hlist to check for pruning) and
121	                   the higher the deduplication factor will be, but
122	                   the slowest the worst case rmap walk could be for
123	                   any given KSM page. Slowing down the rmap_walk
124	                   means there will be higher latency for certain
125	                   virtual memory operations happening during
126	                   swapping, compaction, NUMA balancing and page
127	                   migration, in turn decreasing responsiveness for
128	                   the caller of those virtual memory operations. The
129	                   scheduler latency of other tasks not involved with
130	                   the VM operations doing the rmap walk is not
131	                   affected by this parameter as the rmap walks are
132	                   always schedule friendly themselves.
133	
134	stable_node_chains_prune_millisecs - How frequently to walk the whole
135	                   list of stable_node "dups" linked in the
136	                   stable_node "chains" in order to prune stale
137	                   stable_nodes. Smaller milllisecs values will free
138	                   up the KSM metadata with lower latency, but they
139	                   will make ksmd use more CPU during the scan. This
140	                   only applies to the stable_node chains so it's a
141	                   noop if not a single KSM page hit the
142	                   max_page_sharing yet (there would be no stable_node
143	                   chains in such case).
144	
145	The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
146	
147	pages_shared     - how many shared pages are being used
148	pages_sharing    - how many more sites are sharing them i.e. how much saved
149	pages_unshared   - how many pages unique but repeatedly checked for merging
150	pages_volatile   - how many pages changing too fast to be placed in a tree
151	full_scans       - how many times all mergeable areas have been scanned
152	
153	stable_node_chains - number of stable node chains allocated, this is
154			     effectively the number of KSM pages that hit the
155			     max_page_sharing limit
156	stable_node_dups   - number of stable node dups queued into the
157			     stable_node chains
158	
159	A high ratio of pages_sharing to pages_shared indicates good sharing, but
160	a high ratio of pages_unshared to pages_sharing indicates wasted effort.
161	pages_volatile embraces several different kinds of activity, but a high
162	proportion there would also indicate poor use of madvise MADV_MERGEABLE.
163	
164	The maximum possible page_sharing/page_shared ratio is limited by the
165	max_page_sharing tunable. To increase the ratio max_page_sharing must
166	be increased accordingly.
167	
168	The stable_node_dups/stable_node_chains ratio is also affected by the
169	max_page_sharing tunable, and an high ratio may indicate fragmentation
170	in the stable_node dups, which could be solved by introducing
171	fragmentation algorithms in ksmd which would refile rmap_items from
172	one stable_node dup to another stable_node dup, in order to freeup
173	stable_node "dups" with few rmap_items in them, but that may increase
174	the ksmd CPU usage and possibly slowdown the readonly computations on
175	the KSM pages of the applications.
176	
177	Izik Eidus,
178	Hugh Dickins, 17 Nov 2009
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