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

 =======================
 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
 interface <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``.

         The pages_to_scan value cannot be changed if ``advisor_mode`` has
         been set to scan-time.

         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.

 smart_scan
         Historically KSM checked every candidate page for each scan. It did
         not take into account historic information.  When smart scan is
         enabled, pages that have previously not been de-duplicated get
         skipped. How often these pages are skipped depends on how often
         de-duplication has already been tried and failed. By default this
         optimization is enabled.  The ``pages_skipped`` metric shows how
         effective the setting is.

 advisor_mode
         The ``advisor_mode`` selects the current advisor. Two modes are
         supported: none and scan-time. The default is none. By setting
         ``advisor_mode`` to scan-time, the scan time advisor is enabled.
         The section about ``advisor`` explains in detail how the scan time
         advisor works.

 adivsor_max_cpu
         specifies the upper limit of the cpu percent usage of the ksmd
         background thread. The default is 70.

 advisor_target_scan_time
         specifies the target scan time in seconds to scan all the candidate
         pages. The default value is 200 seconds.

 advisor_min_pages_to_scan
         specifies the lower limit of the ``pages_to_scan`` parameter of the
         scan time advisor. The default is 500.

 adivsor_max_pages_to_scan
         specifies the upper limit of the ``pages_to_scan`` parameter of the
         scan time advisor. The default is 30000.

 The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:

 general_profit
         how effective is KSM. The calculation is explained below.
 pages_scanned
         how many pages are being scanned for 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
 pages_skipped
         how many pages did the "smart" page scanning algorithm skip
 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
 ksm_zero_pages
         how many zero pages that are still mapped into processes were mapped by
         KSM when deduplicating.

 When ``use_zero_pages`` is/was enabled, the sum of ``pages_sharing`` +
 ``ksm_zero_pages`` represents the actual number of pages saved by KSM.
 if ``use_zero_pages`` has never been enabled, ``ksm_zero_pages`` is 0.

 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.

 Monitoring KSM profit
 =====================

 KSM can save memory by merging identical pages, but also can consume
 additional memory, because it needs to generate a number of rmap_items to
 save each scanned page's brief rmap information. Some of these pages may
 be merged, but some may not be abled to be merged after being checked
 several times, which are unprofitable memory consumed.

 1) How to determine whether KSM save memory or consume memory in system-wide
    range? Here is a simple approximate calculation for reference::

 	general_profit =~ ksm_saved_pages * sizeof(page) - (all_rmap_items) *
 			  sizeof(rmap_item);

    where ksm_saved_pages equals to the sum of ``pages_sharing`` +
    ``ksm_zero_pages`` of the system, and all_rmap_items can be easily
    obtained by summing ``pages_sharing``, ``pages_shared``, ``pages_unshared``
    and ``pages_volatile``.

 2) The KSM profit inner a single process can be similarly obtained by the
    following approximate calculation::

 	process_profit =~ ksm_saved_pages * sizeof(page) -
 			  ksm_rmap_items * sizeof(rmap_item).

    where ksm_saved_pages equals to the sum of ``ksm_merging_pages`` and
    ``ksm_zero_pages``, both of which are shown under the directory
    ``/proc/<pid>/ksm_stat``, and ksm_rmap_items is also shown in
    ``/proc/<pid>/ksm_stat``. The process profit is also shown in
    ``/proc/<pid>/ksm_stat`` as ksm_process_profit.

 From the perspective of application, a high ratio of ``ksm_rmap_items`` to
 ``ksm_merging_pages`` means a bad madvise-applied policy, so developers or
 administrators have to rethink how to change madvise policy. Giving an example
 for reference, a page's size is usually 4K, and the rmap_item's size is
 separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
 so if the ``ksm_rmap_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU
 or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
 because the ksm profit is approximately zero or negative.

 Monitoring KSM events
 =====================

 There are some counters in /proc/vmstat that may be used to monitor KSM events.
 KSM might help save memory, it's a tradeoff by may suffering delay on KSM COW
 or on swapping in copy. Those events could help users evaluate whether or how
 to use KSM. For example, if cow_ksm increases too fast, user may decrease the
 range of madvise(, , MADV_MERGEABLE).

 cow_ksm
 	is incremented every time a KSM page triggers copy on write (COW)
 	when users try to write to a KSM page, we have to make a copy.

 ksm_swpin_copy
 	is incremented every time a KSM page is copied when swapping in
 	note that KSM page might be copied when swapping in because do_swap_page()
 	cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.

 Advisor
 =======

 The number of candidate pages for KSM is dynamic. It can be often observed
 that during the startup of an application more candidate pages need to be
 processed. Without an advisor the ``pages_to_scan`` parameter needs to be
 sized for the maximum number of candidate pages. The scan time advisor can
 changes the ``pages_to_scan`` parameter based on demand.

 The advisor can be enabled, so KSM can automatically adapt to changes in the
 number of candidate pages to scan. Two advisors are implemented: none and
 scan-time. With none, no advisor is enabled. The default is none.

 The scan time advisor changes the ``pages_to_scan`` parameter based on the
 observed scan times. The possible values for the ``pages_to_scan`` parameter is
 limited by the ``advisor_max_cpu`` parameter. In addition there is also the
 ``advisor_target_scan_time`` parameter. This parameter sets the target time to
 scan all the KSM candidate pages. The parameter ``advisor_target_scan_time``
 decides how aggressive the scan time advisor scans candidate pages. Lower
 values make the scan time advisor to scan more aggressively. This is the most
 important parameter for the configuration of the scan time advisor.

 The initial value and the maximum value can be changed with
 ``advisor_min_pages_to_scan`` and ``advisor_max_pages_to_scan``. The default
 values are sufficient for most workloads and use cases.

 The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.


 --
 Izik Eidus,
 Hugh Dickins, 17 Nov 2009
	=======================
	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
	interface <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``.

	The pages_to_scan value cannot be changed if ``advisor_mode`` has
	been set to scan-time.

	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.

	smart_scan
	Historically KSM checked every candidate page for each scan. It did
	not take into account historic information. When smart scan is
	enabled, pages that have previously not been de-duplicated get
	skipped. How often these pages are skipped depends on how often
	de-duplication has already been tried and failed. By default this
	optimization is enabled. The ``pages_skipped`` metric shows how
	effective the setting is.

	advisor_mode
	The ``advisor_mode`` selects the current advisor. Two modes are
	supported: none and scan-time. The default is none. By setting
	``advisor_mode`` to scan-time, the scan time advisor is enabled.
	The section about ``advisor`` explains in detail how the scan time
	advisor works.

	adivsor_max_cpu
	specifies the upper limit of the cpu percent usage of the ksmd
	background thread. The default is 70.

	advisor_target_scan_time
	specifies the target scan time in seconds to scan all the candidate
	pages. The default value is 200 seconds.

	advisor_min_pages_to_scan
	specifies the lower limit of the ``pages_to_scan`` parameter of the
	scan time advisor. The default is 500.

	adivsor_max_pages_to_scan
	specifies the upper limit of the ``pages_to_scan`` parameter of the
	scan time advisor. The default is 30000.

	The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:

	general_profit
	how effective is KSM. The calculation is explained below.
	pages_scanned
	how many pages are being scanned for 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
	pages_skipped
	how many pages did the "smart" page scanning algorithm skip
	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
	ksm_zero_pages
	how many zero pages that are still mapped into processes were mapped by
	KSM when deduplicating.

	When ``use_zero_pages`` is/was enabled, the sum of ``pages_sharing`` +
	``ksm_zero_pages`` represents the actual number of pages saved by KSM.
	if ``use_zero_pages`` has never been enabled, ``ksm_zero_pages`` is 0.

	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.

	Monitoring KSM profit
	=====================

	KSM can save memory by merging identical pages, but also can consume
	additional memory, because it needs to generate a number of rmap_items to
	save each scanned page's brief rmap information. Some of these pages may
	be merged, but some may not be abled to be merged after being checked
	several times, which are unprofitable memory consumed.

	1) How to determine whether KSM save memory or consume memory in system-wide
	range? Here is a simple approximate calculation for reference::

	general_profit =~ ksm_saved_pages * sizeof(page) - (all_rmap_items) *
	sizeof(rmap_item);

	where ksm_saved_pages equals to the sum of ``pages_sharing`` +
	``ksm_zero_pages`` of the system, and all_rmap_items can be easily
	obtained by summing ``pages_sharing``, ``pages_shared``, ``pages_unshared``
	and ``pages_volatile``.

	2) The KSM profit inner a single process can be similarly obtained by the
	following approximate calculation::

	process_profit =~ ksm_saved_pages * sizeof(page) -
	ksm_rmap_items * sizeof(rmap_item).

	where ksm_saved_pages equals to the sum of ``ksm_merging_pages`` and
	``ksm_zero_pages``, both of which are shown under the directory
	``/proc/<pid>/ksm_stat``, and ksm_rmap_items is also shown in
	``/proc/<pid>/ksm_stat``. The process profit is also shown in
	``/proc/<pid>/ksm_stat`` as ksm_process_profit.

	From the perspective of application, a high ratio of ``ksm_rmap_items`` to
	``ksm_merging_pages`` means a bad madvise-applied policy, so developers or
	administrators have to rethink how to change madvise policy. Giving an example
	for reference, a page's size is usually 4K, and the rmap_item's size is
	separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
	so if the ``ksm_rmap_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU
	or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
	because the ksm profit is approximately zero or negative.

	Monitoring KSM events
	=====================

	There are some counters in /proc/vmstat that may be used to monitor KSM events.
	KSM might help save memory, it's a tradeoff by may suffering delay on KSM COW
	or on swapping in copy. Those events could help users evaluate whether or how
	to use KSM. For example, if cow_ksm increases too fast, user may decrease the
	range of madvise(, , MADV_MERGEABLE).

	cow_ksm
	is incremented every time a KSM page triggers copy on write (COW)
	when users try to write to a KSM page, we have to make a copy.

	ksm_swpin_copy
	is incremented every time a KSM page is copied when swapping in
	note that KSM page might be copied when swapping in because do_swap_page()
	cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.

	Advisor
	=======

	The number of candidate pages for KSM is dynamic. It can be often observed
	that during the startup of an application more candidate pages need to be
	processed. Without an advisor the ``pages_to_scan`` parameter needs to be
	sized for the maximum number of candidate pages. The scan time advisor can
	changes the ``pages_to_scan`` parameter based on demand.

	The advisor can be enabled, so KSM can automatically adapt to changes in the
	number of candidate pages to scan. Two advisors are implemented: none and
	scan-time. With none, no advisor is enabled. The default is none.

	The scan time advisor changes the ``pages_to_scan`` parameter based on the
	observed scan times. The possible values for the ``pages_to_scan`` parameter is
	limited by the ``advisor_max_cpu`` parameter. In addition there is also the
	``advisor_target_scan_time`` parameter. This parameter sets the target time to
	scan all the KSM candidate pages. The parameter ``advisor_target_scan_time``
	decides how aggressive the scan time advisor scans candidate pages. Lower
	values make the scan time advisor to scan more aggressively. This is the most
	important parameter for the configuration of the scan time advisor.

	The initial value and the maximum value can be changed with
	``advisor_min_pages_to_scan`` and ``advisor_max_pages_to_scan``. The default
	values are sufficient for most workloads and use cases.

	The ``pages_to_scan`` parameter is re-calculated after a scan has been completed.


	--
	Izik Eidus,
	Hugh Dickins, 17 Nov 2009