Documentation/vm/ksm.txt

   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 The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
 102
 103 pages_shared     - how many shared pages are being used
 104 pages_sharing    - how many more sites are sharing them i.e. how much saved
 105 pages_unshared   - how many pages unique but repeatedly checked for merging
 106 pages_volatile   - how many pages changing too fast to be placed in a tree
 107 full_scans       - how many times all mergeable areas have been scanned
 108
 109 A high ratio of pages_sharing to pages_shared indicates good sharing, but
 110 a high ratio of pages_unshared to pages_sharing indicates wasted effort.
 111 pages_volatile embraces several different kinds of activity, but a high
 112 proportion there would also indicate poor use of madvise MADV_MERGEABLE.
 113
 114 Izik Eidus,
 115 Hugh Dickins, 17 Nov 2009