Documentation/vm/ksm.rst - linux - Git at Google

 .. _ksm:

 =======================
 Kernel Samepage Merging
 =======================

 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)

 max_page_sharing
         Maximum sharing allowed for each KSM page. This enforces a
         deduplication limit to avoid the virtual memory rmap lists to
         grow too large. The minimum value is 2 as a newly created KSM
         page will have at least two sharers. The rmap walk has O(N)
         complexity where N is the number of rmap_items (i.e. virtual
         mappings) that are sharing the page, which is in turn capped
         by max_page_sharing. So this effectively spread the the linear
         O(N) computational complexity from rmap walk context over
         different KSM pages. The ksmd walk over the stable_node
         "chains" is also O(N), but N is the number of stable_node
         "dups", not the number of rmap_items, so it has not a
         significant impact on ksmd performance. In practice the best
         stable_node "dup" candidate will be kept and found at the head
         of the "dups" list. The higher this value the faster KSM will
         merge the memory (because there will be fewer stable_node dups
         queued into the stable_node chain->hlist to check for pruning)
         and the higher the deduplication factor will be, but the
         slowest the worst case rmap walk could be for any given KSM
         page. Slowing down the rmap_walk means there will be higher
         latency for certain virtual memory operations happening during
         swapping, compaction, NUMA balancing and page migration, in
         turn decreasing responsiveness for the caller of those virtual
         memory operations. The scheduler latency of other tasks not
         involved with the VM operations doing the rmap walk is not
         affected by this parameter as the rmap walks are always
         schedule friendly themselves.

 stable_node_chains_prune_millisecs
         How frequently to walk the whole list of stable_node "dups"
         linked in the stable_node "chains" in order to prune stale
         stable_nodes. Smaller milllisecs values will free up the KSM
         metadata with lower latency, but they will make ksmd use more
         CPU during the scan. This only applies to the stable_node
         chains so it's a noop if not a single KSM page hit the
         max_page_sharing yet (there would be no stable_node chains in
         such case).

 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
 stable_node_chains
         number of stable node chains allocated, this is effectively
         the number of KSM pages that hit the max_page_sharing limit
 stable_node_dups
         number of stable node dups queued into the stable_node chains

 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.

 The maximum possible page_sharing/page_shared ratio is limited by the
 max_page_sharing tunable. To increase the ratio max_page_sharing must
 be increased accordingly.

 The stable_node_dups/stable_node_chains ratio is also affected by the
 max_page_sharing tunable, and an high ratio may indicate fragmentation
 in the stable_node dups, which could be solved by introducing
 fragmentation algorithms in ksmd which would refile rmap_items from
 one stable_node dup to another stable_node dup, in order to freeup
 stable_node "dups" with few rmap_items in them, but that may increase
 the ksmd CPU usage and possibly slowdown the readonly computations on
 the KSM pages of the applications.

 Izik Eidus,
 Hugh Dickins, 17 Nov 2009
	.. _ksm:

	=======================
	Kernel Samepage Merging
	=======================

	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)

	max_page_sharing
	Maximum sharing allowed for each KSM page. This enforces a
	deduplication limit to avoid the virtual memory rmap lists to
	grow too large. The minimum value is 2 as a newly created KSM
	page will have at least two sharers. The rmap walk has O(N)
	complexity where N is the number of rmap_items (i.e. virtual
	mappings) that are sharing the page, which is in turn capped
	by max_page_sharing. So this effectively spread the the linear
	O(N) computational complexity from rmap walk context over
	different KSM pages. The ksmd walk over the stable_node
	"chains" is also O(N), but N is the number of stable_node
	"dups", not the number of rmap_items, so it has not a
	significant impact on ksmd performance. In practice the best
	stable_node "dup" candidate will be kept and found at the head
	of the "dups" list. The higher this value the faster KSM will
	merge the memory (because there will be fewer stable_node dups
	queued into the stable_node chain->hlist to check for pruning)
	and the higher the deduplication factor will be, but the
	slowest the worst case rmap walk could be for any given KSM
	page. Slowing down the rmap_walk means there will be higher
	latency for certain virtual memory operations happening during
	swapping, compaction, NUMA balancing and page migration, in
	turn decreasing responsiveness for the caller of those virtual
	memory operations. The scheduler latency of other tasks not
	involved with the VM operations doing the rmap walk is not
	affected by this parameter as the rmap walks are always
	schedule friendly themselves.

	stable_node_chains_prune_millisecs
	How frequently to walk the whole list of stable_node "dups"
	linked in the stable_node "chains" in order to prune stale
	stable_nodes. Smaller milllisecs values will free up the KSM
	metadata with lower latency, but they will make ksmd use more
	CPU during the scan. This only applies to the stable_node
	chains so it's a noop if not a single KSM page hit the
	max_page_sharing yet (there would be no stable_node chains in
	such case).

	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
	stable_node_chains
	number of stable node chains allocated, this is effectively
	the number of KSM pages that hit the max_page_sharing limit
	stable_node_dups
	number of stable node dups queued into the stable_node chains

	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.

	The maximum possible page_sharing/page_shared ratio is limited by the
	max_page_sharing tunable. To increase the ratio max_page_sharing must
	be increased accordingly.

	The stable_node_dups/stable_node_chains ratio is also affected by the
	max_page_sharing tunable, and an high ratio may indicate fragmentation
	in the stable_node dups, which could be solved by introducing
	fragmentation algorithms in ksmd which would refile rmap_items from
	one stable_node dup to another stable_node dup, in order to freeup
	stable_node "dups" with few rmap_items in them, but that may increase
	the ksmd CPU usage and possibly slowdown the readonly computations on
	the KSM pages of the applications.

	Izik Eidus,
	Hugh Dickins, 17 Nov 2009