Documentation/admin-guide/mm/ksm.rst - linux - Git at Google

 .. _admin_guide_ksm:

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

 Overview
 ========

 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 https://lwn.net/Articles/330589/

 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.

 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). The amount of pages that KSM daemon scans in a single pass
 and the time between the passes are configured using :ref:`sysfs
 intraface <ksm_sysfs>`

 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).

 Controlling KSM with madvise
 ============================

 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
 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/admin-guide/sysctl/vm.rst).

 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.

 .. _ksm_sysfs:

 KSM daemon sysfs interface
 ==========================

 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 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 the 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 to 0 to stop ksmd from running but keep merged pages,
         * set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
         * set to 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 high latency for virtual memory
         operations that involve traversal of the virtual mappings that
         share the KSM page. The minimum value is 2 as a newly created
         KSM page will have at least two sharers. The higher this value
         the faster KSM will merge the memory and the higher the
         deduplication factor will be, but the slower the worst case
         virtual mappings traversal could be for any given KSM
         page. Slowing down this traversal 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 virtual mappings
         traversal is not affected by this parameter as these
         traversals are always schedule friendly themselves.

 stable_node_chains_prune_millisecs
         specifies how frequently KSM checks the metadata of the pages
         that hit the deduplication limit for stale information.
         Smaller milllisecs values will free up the KSM metadata with
         lower latency, but they will make ksmd use more CPU during the
         scan. It's a noop if not a single KSM page hit the
         ``max_page_sharing`` yet.

 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
         the number of KSM pages that hit the ``max_page_sharing`` limit
 stable_node_dups
         number of duplicated KSM pages

 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 ``pages_sharing/pages_shared`` ratio is limited by the
 ``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
 be increased accordingly.

 --
 Izik Eidus,
 Hugh Dickins, 17 Nov 2009
	.. _admin_guide_ksm:

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

	Overview
	========

	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 https://lwn.net/Articles/330589/

	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.

	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). The amount of pages that KSM daemon scans in a single pass
	and the time between the passes are configured using :ref:`sysfs
	intraface <ksm_sysfs>`

	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).

	Controlling KSM with madvise
	============================

	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
	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/admin-guide/sysctl/vm.rst).

	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.

	.. _ksm_sysfs:

	KSM daemon sysfs interface
	==========================

	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 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 the 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 to 0 to stop ksmd from running but keep merged pages,
	* set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
	* set to 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 high latency for virtual memory
	operations that involve traversal of the virtual mappings that
	share the KSM page. The minimum value is 2 as a newly created
	KSM page will have at least two sharers. The higher this value
	the faster KSM will merge the memory and the higher the
	deduplication factor will be, but the slower the worst case
	virtual mappings traversal could be for any given KSM
	page. Slowing down this traversal 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 virtual mappings
	traversal is not affected by this parameter as these
	traversals are always schedule friendly themselves.

	stable_node_chains_prune_millisecs
	specifies how frequently KSM checks the metadata of the pages
	that hit the deduplication limit for stale information.
	Smaller milllisecs values will free up the KSM metadata with
	lower latency, but they will make ksmd use more CPU during the
	scan. It's a noop if not a single KSM page hit the
	``max_page_sharing`` yet.

	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
	the number of KSM pages that hit the ``max_page_sharing`` limit
	stable_node_dups
	number of duplicated KSM pages

	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 ``pages_sharing/pages_shared`` ratio is limited by the
	``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
	be increased accordingly.

	--
	Izik Eidus,
	Hugh Dickins, 17 Nov 2009