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authorHugh Dickins <hugh.dickins@tiscali.co.uk>2009-09-21 20:02:24 -0400
committerLinus Torvalds <torvalds@linux-foundation.org>2009-09-22 10:17:33 -0400
commit7701c9c0f54feb682d0cefa2ae1f4a1e00e0ba09 (patch)
tree50f98c7bf7071f1559c529b0c963a9dc86c54e00 /Documentation/vm
parent2ffd8679c8e4ec226718bff58b50b226dd477015 (diff)
ksm: add some documentation
Add Documentation/vm/ksm.txt: how to use the Kernel Samepage Merging feature Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk> Cc: Michael Kerrisk <mtk.manpages@googlemail.com> Cc: Randy Dunlap <randy.dunlap@oracle.com> Acked-by: Izik Eidus <ieidus@redhat.com> Cc: Andrea Arcangeli <aarcange@redhat.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'Documentation/vm')
-rw-r--r--Documentation/vm/00-INDEX2
-rw-r--r--Documentation/vm/ksm.txt89
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diff --git a/Documentation/vm/00-INDEX b/Documentation/vm/00-INDEX
index 2f77ced35df7..f80a44944874 100644
--- a/Documentation/vm/00-INDEX
+++ b/Documentation/vm/00-INDEX
@@ -6,6 +6,8 @@ balance
6 - various information on memory balancing. 6 - various information on memory balancing.
7hugetlbpage.txt 7hugetlbpage.txt
8 - a brief summary of hugetlbpage support in the Linux kernel. 8 - a brief summary of hugetlbpage support in the Linux kernel.
9ksm.txt
10 - how to use the Kernel Samepage Merging feature.
9locking 11locking
10 - info on how locking and synchronization is done in the Linux vm code. 12 - info on how locking and synchronization is done in the Linux vm code.
11numa 13numa
diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt
new file mode 100644
index 000000000000..72a22f65960e
--- /dev/null
+++ b/Documentation/vm/ksm.txt
@@ -0,0 +1,89 @@
1How to use the Kernel Samepage Merging feature
2----------------------------------------------
3
4KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
5added to the Linux kernel in 2.6.32. See mm/ksm.c for its implementation,
6and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/
7
8The KSM daemon ksmd periodically scans those areas of user memory which
9have been registered with it, looking for pages of identical content which
10can be replaced by a single write-protected page (which is automatically
11copied if a process later wants to update its content).
12
13KSM was originally developed for use with KVM (where it was known as
14Kernel Shared Memory), to fit more virtual machines into physical memory,
15by sharing the data common between them. But it can be useful to any
16application which generates many instances of the same data.
17
18KSM only merges anonymous (private) pages, never pagecache (file) pages.
19KSM's merged pages are at present locked into kernel memory for as long
20as they are shared: so cannot be swapped out like the user pages they
21replace (but swapping KSM pages should follow soon in a later release).
22
23KSM only operates on those areas of address space which an application
24has advised to be likely candidates for merging, by using the madvise(2)
25system call: int madvise(addr, length, MADV_MERGEABLE).
26
27The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel
28that advice and restore unshared pages: whereupon KSM unmerges whatever
29it merged in that range. Note: this unmerging call may suddenly require
30more memory than is available - possibly failing with EAGAIN, but more
31probably arousing the Out-Of-Memory killer.
32
33If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
34and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
35built with CONFIG_KSM=y, those calls will normally succeed: even if the
36the KSM daemon is not currently running, MADV_MERGEABLE still registers
37the range for whenever the KSM daemon is started; even if the range
38cannot contain any pages which KSM could actually merge; even if
39MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
40
41Like other madvise calls, they are intended for use on mapped areas of
42the user address space: they will report ENOMEM if the specified range
43includes unmapped gaps (though working on the intervening mapped areas),
44and might fail with EAGAIN if not enough memory for internal structures.
45
46Applications should be considerate in their use of MADV_MERGEABLE,
47restricting its use to areas likely to benefit. KSM's scans may use
48a lot of processing power, and its kernel-resident pages are a limited
49resource. Some installations will disable KSM for these reasons.
50
51The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
52readable by all but writable only by root:
53
54max_kernel_pages - set to maximum number of kernel pages that KSM may use
55 e.g. "echo 2000 > /sys/kernel/mm/ksm/max_kernel_pages"
56 Value 0 imposes no limit on the kernel pages KSM may use;
57 but note that any process using MADV_MERGEABLE can cause
58 KSM to allocate these pages, unswappable until it exits.
59 Default: 2000 (chosen for demonstration purposes)
60
61pages_to_scan - how many present pages to scan before ksmd goes to sleep
62 e.g. "echo 200 > /sys/kernel/mm/ksm/pages_to_scan"
63 Default: 200 (chosen for demonstration purposes)
64
65sleep_millisecs - how many milliseconds ksmd should sleep before next scan
66 e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs"
67 Default: 20 (chosen for demonstration purposes)
68
69run - set 0 to stop ksmd from running but keep merged pages,
70 set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
71 set 2 to stop ksmd and unmerge all pages currently merged,
72 but leave mergeable areas registered for next run
73 Default: 1 (for immediate use by apps which register)
74
75The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
76
77pages_shared - how many shared unswappable kernel pages KSM is using
78pages_sharing - how many more sites are sharing them i.e. how much saved
79pages_unshared - how many pages unique but repeatedly checked for merging
80pages_volatile - how many pages changing too fast to be placed in a tree
81full_scans - how many times all mergeable areas have been scanned
82
83A high ratio of pages_sharing to pages_shared indicates good sharing, but
84a high ratio of pages_unshared to pages_sharing indicates wasted effort.
85pages_volatile embraces several different kinds of activity, but a high
86proportion there would also indicate poor use of madvise MADV_MERGEABLE.
87
88Izik Eidus,
89Hugh Dickins, 30 July 2009