linux/Documentation/vm/ksm.txt
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   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 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).
  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

  v2.6.18.2
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