1How to use the Kernel Samepage Merging feature
   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 and
   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).
  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.
  18KSM only merges anonymous (private) pages, never pagecache (file) pages.
  19KSM's merged pages were originally locked into kernel memory, but can now
  20be swapped out just like other user pages (but sharing is broken when they
  21are swapped back in: ksmd must rediscover their identity and merge again).
  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).
  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.
  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.
  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.
  46Applications should be considerate in their use of MADV_MERGEABLE,
  47restricting its use to areas likely to benefit.  KSM's scans may use a lot
  48of processing power: some installations will disable KSM for that reason.
  50The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
  51readable by all but writable only by root:
  53pages_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)
  57sleep_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)
  61merge_across_nodes - specifies if pages from different numa nodes can be merged.
  62                   When set to 0, ksm merges only pages which physically
  63                   reside in the memory area of same NUMA node. That brings
  64                   lower latency to access of shared pages. Systems with more
  65                   nodes, at significant NUMA distances, are likely to benefit
  66                   from the lower latency of setting 0. Smaller systems, which
  67                   need to minimize memory usage, are likely to benefit from
  68                   the greater sharing of setting 1 (default). You may wish to
  69                   compare how your system performs under each setting, before
  70                   deciding on which to use. merge_across_nodes setting can be
  71                   changed only when there are no ksm shared pages in system:
  72                   set run 2 to unmerge pages first, then to 1 after changing
  73                   merge_across_nodes, to remerge according to the new setting.
  74                   Default: 1 (merging across nodes as in earlier releases)
  76run              - set 0 to stop ksmd from running but keep merged pages,
  77                   set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
  78                   set 2 to stop ksmd and unmerge all pages currently merged,
  79                         but leave mergeable areas registered for next run
  80                   Default: 0 (must be changed to 1 to activate KSM,
  81                               except if CONFIG_SYSFS is disabled)
  83The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
  85pages_shared     - how many shared pages are being used
  86pages_sharing    - how many more sites are sharing them i.e. how much saved
  87pages_unshared   - how many pages unique but repeatedly checked for merging
  88pages_volatile   - how many pages changing too fast to be placed in a tree
  89full_scans       - how many times all mergeable areas have been scanned
  91A high ratio of pages_sharing to pages_shared indicates good sharing, but
  92a high ratio of pages_unshared to pages_sharing indicates wasted effort.
  93pages_volatile embraces several different kinds of activity, but a high
  94proportion there would also indicate poor use of madvise MADV_MERGEABLE.
  96Izik Eidus,
  97Hugh Dickins, 17 Nov 2009
  98 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.