docs/vm: ksm: split userspace interface to admin-guide/mm/ksm.rst (c9161088) · Commits · e / devices / android_kernel_fairphone_FP5

Documentation/admin-guide/mm/index.rst

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		hugetlbpage
		idle_page_tracking
		ksm
		pagemap
		soft-dirty
		userfaultfd

Documentation/admin-guide/mm/ksm.rst

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		.. _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 http://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
		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.

		.. _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

Documentation/vm/ksm.rst

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Original line number	Diff line number	Diff line
		@@ -4,185 +4,11 @@
		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 http://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
		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.

		.. _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.
		The userspace interface of KSM is described in :ref:`Documentation/admin-guide/mm/ksm.rst <admin_guide_ksm>`

		Design
		======