Documentation/vm/ksm.txt - kernel/msm-4.19 - Gitiles

 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 http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/

 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.

 If a region of memory must be split into at least one new MADV_MERGEABLE
 or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
 will exceed vm.max_map_count (see Documentation/sysctl/vm.txt).

 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)

 merge_across_nodes - specifies if pages from different numa nodes can be merged.
                    When set to 0, ksm merges only pages which physically
                    reside in the memory area of same NUMA node. That brings
                    lower latency to access of shared pages. Systems with more
                    nodes, at significant NUMA distances, are likely to benefit
                    from the lower latency of setting 0. Smaller systems, which
                    need to minimize memory usage, are likely to benefit from
                    the greater sharing of setting 1 (default). You may wish to
                    compare how your system performs under each setting, before
                    deciding on which to use. merge_across_nodes setting can be
                    changed only when there are no ksm shared pages in system:
                    set run 2 to unmerge pages first, then to 1 after changing
                    merge_across_nodes, to remerge according to the new setting.
                    Default: 1 (merging across nodes as in earlier releases)

 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)

 use_zero_pages   - specifies whether empty pages (i.e. allocated pages
                    that only contain zeroes) should be treated specially.
                    When set to 1, empty pages are merged with the kernel
                    zero page(s) instead of with each other as it would
                    happen normally. This can improve the performance on
                    architectures with coloured zero pages, depending on
                    the workload. Care should be taken when enabling this
                    setting, as it can potentially degrade the performance
                    of KSM for some workloads, for example if the checksums
                    of pages candidate for merging match the checksum of
                    an empty page. This setting can be changed at any time,
                    it is only effective for pages merged after the change.
                    Default: 0 (normal KSM behaviour as in earlier releases)

 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
	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 http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/

	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.

	If a region of memory must be split into at least one new MADV_MERGEABLE
	or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
	will exceed vm.max_map_count (see Documentation/sysctl/vm.txt).

	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)

	merge_across_nodes - specifies if pages from different numa nodes can be merged.
	When set to 0, ksm merges only pages which physically
	reside in the memory area of same NUMA node. That brings
	lower latency to access of shared pages. Systems with more
	nodes, at significant NUMA distances, are likely to benefit
	from the lower latency of setting 0. Smaller systems, which
	need to minimize memory usage, are likely to benefit from
	the greater sharing of setting 1 (default). You may wish to
	compare how your system performs under each setting, before
	deciding on which to use. merge_across_nodes setting can be
	changed only when there are no ksm shared pages in system:
	set run 2 to unmerge pages first, then to 1 after changing
	merge_across_nodes, to remerge according to the new setting.
	Default: 1 (merging across nodes as in earlier releases)

	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)

	use_zero_pages - specifies whether empty pages (i.e. allocated pages
	that only contain zeroes) should be treated specially.
	When set to 1, empty pages are merged with the kernel
	zero page(s) instead of with each other as it would
	happen normally. This can improve the performance on
	architectures with coloured zero pages, depending on
	the workload. Care should be taken when enabling this
	setting, as it can potentially degrade the performance
	of KSM for some workloads, for example if the checksums
	of pages candidate for merging match the checksum of
	an empty page. This setting can be changed at any time,
	it is only effective for pages merged after the change.
	Default: 0 (normal KSM behaviour as in earlier releases)

	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