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-How to use the Kernel Samepage Merging feature
-KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
-added to the Linux kernel in 2.6.32. See mm/ksm.c for its implementation,
-and and
-The KSM daemon ksmd periodically scans those areas of user memory which
-have been registered with it, looking for pages of identical content which
-can be replaced by a single write-protected page (which is automatically
-copied if a process later wants to update its content).
-KSM was originally developed for use with KVM (where it was known as
-Kernel Shared Memory), to fit more virtual machines into physical memory,
-by sharing the data common between them. But it can be useful to any
-application which generates many instances of the same data.
-KSM only merges anonymous (private) pages, never pagecache (file) pages.
-KSM's merged pages were originally locked into kernel memory, but can now
-be swapped out just like other user pages (but sharing is broken when they
-are swapped back in: ksmd must rediscover their identity and merge again).
-KSM only operates on those areas of address space which an application
-has advised to be likely candidates for merging, by using the madvise(2)
-system call: int madvise(addr, length, MADV_MERGEABLE).
-The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel
-that advice and restore unshared pages: whereupon KSM unmerges whatever
-it merged in that range. Note: this unmerging call may suddenly require
-more memory than is available - possibly failing with EAGAIN, but more
-probably arousing the Out-Of-Memory killer.
-If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
-and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
-built with CONFIG_KSM=y, those calls will normally succeed: even if the
-the KSM daemon is not currently running, MADV_MERGEABLE still registers
-the range for whenever the KSM daemon is started; even if the range
-cannot contain any pages which KSM could actually merge; even if
-MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
-Like other madvise calls, they are intended for use on mapped areas of
-the user address space: they will report ENOMEM if the specified range
-includes unmapped gaps (though working on the intervening mapped areas),
-and might fail with EAGAIN if not enough memory for internal structures.
-Applications should be considerate in their use of MADV_MERGEABLE,
-restricting its use to areas likely to benefit. KSM's scans may use a lot
-of processing power: some installations will disable KSM for that reason.
-The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
-readable by all but writable only by root:
-pages_to_scan - how many present pages to scan before ksmd goes to sleep
- e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan"
- Default: 100 (chosen for demonstration purposes)
-sleep_millisecs - how many milliseconds ksmd should sleep before next scan
- e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs"
- Default: 20 (chosen for demonstration purposes)
-run - set 0 to stop ksmd from running but keep merged pages,
- set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
- set 2 to stop ksmd and unmerge all pages currently merged,
- but leave mergeable areas registered for next run
- Default: 0 (must be changed to 1 to activate KSM,
- except if CONFIG_SYSFS is disabled)
-The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
-pages_shared - how many shared pages are being used
-pages_sharing - how many more sites are sharing them i.e. how much saved
-pages_unshared - how many pages unique but repeatedly checked for merging
-pages_volatile - how many pages changing too fast to be placed in a tree
-full_scans - how many times all mergeable areas have been scanned
-A high ratio of pages_sharing to pages_shared indicates good sharing, but
-a high ratio of pages_unshared to pages_sharing indicates wasted effort.
-pages_volatile embraces several different kinds of activity, but a high
-proportion there would also indicate poor use of madvise MADV_MERGEABLE.
-Izik Eidus,
-Hugh Dickins, 17 Nov 2009