docs/vm: ksm: (mostly) formatting updates (db12c00f) · Commits · e / devices / android_kernel_fairphone_FP4

Documentation/vm/ksm.rst

+69 −41

Original line number	Original line	Diff line number	Diff line
	@@ -4,34 +4,50 @@
	Kernel Samepage Merging		Kernel Samepage Merging
	=======================		=======================

			Overview
			========

	KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,		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,		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/		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		KSM was originally developed for use with KVM (where it was known as
	Kernel Shared Memory), to fit more virtual machines into physical memory,		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		by sharing the data common between them. But it can be useful to any
	application which generates many instances of the same data.		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 only merges anonymous (private) pages, never pagecache (file) pages.
	KSM's merged pages were originally locked into kernel memory, but can now		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		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).		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		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)		has advised to be likely candidates for merging, by using the madvise(2)
	system call: int madvise(addr, length, MADV_MERGEABLE).		system call::

			int madvise(addr, length, MADV_MERGEABLE)

			The app may call

			::

			int madvise(addr, length, MADV_UNMERGEABLE)

	The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel		to cancel that advice and restore unshared pages: whereupon KSM
	that advice and restore unshared pages: whereupon KSM unmerges whatever		unmerges whatever it merged in that range. Note: this unmerging call
	it merged in that range. Note: this unmerging call may suddenly require		may suddenly require more memory than is available - possibly failing
	more memory than is available - possibly failing with EAGAIN, but more		with EAGAIN, but more probably arousing the Out-Of-Memory killer.
	probably arousing the Out-Of-Memory killer.

	If KSM is not configured into the running kernel, madvise MADV_MERGEABLE		If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
	and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was		and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
	@@ -43,7 +59,7 @@ 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		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		or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
	will exceed vm.max_map_count (see Documentation/sysctl/vm.txt).		will exceed ``vm.max_map_count`` (see Documentation/sysctl/vm.txt).

	Like other madvise calls, they are intended for use on mapped areas of		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		the user address space: they will report ENOMEM if the specified range
	@@ -54,21 +70,28 @@ 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		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.		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/``,		The KSM daemon is controlled by sysfs files in ``/sys/kernel/mm/ksm/``,
	readable by all but writable only by root:		readable by all but writable only by root:

	pages_to_scan		pages_to_scan
	how many present pages to scan before ksmd goes to sleep		how many pages to scan before ksmd goes to sleep
	e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan`` Default: 100		e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
	(chosen for demonstration purposes)
			Default: 100 (chosen for demonstration purposes)

	sleep_millisecs		sleep_millisecs
	how many milliseconds ksmd should sleep before next scan		how many milliseconds ksmd should sleep before next scan
	e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs`` Default: 20		e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs``
	(chosen for demonstration purposes)
			Default: 20 (chosen for demonstration purposes)

	merge_across_nodes		merge_across_nodes
	specifies if pages from different numa nodes can be merged.		specifies if pages from different NUMA nodes can be merged.
	When set to 0, ksm merges only pages which physically reside		When set to 0, ksm merges only pages which physically reside
	in the memory area of same NUMA node. That brings lower		in the memory area of same NUMA node. That brings lower
	latency to access of shared pages. Systems with more nodes, at		latency to access of shared pages. Systems with more nodes, at
	@@ -77,19 +100,21 @@ merge_across_nodes
	minimize memory usage, are likely to benefit from the greater		minimize memory usage, are likely to benefit from the greater
	sharing of setting 1 (default). You may wish to compare how		sharing of setting 1 (default). You may wish to compare how
	your system performs under each setting, before deciding on		your system performs under each setting, before deciding on
	which to use. merge_across_nodes setting can be changed only		which to use. ``merge_across_nodes`` setting can be changed only
	when there are no ksm shared pages in system: set run 2 to		when there are no ksm shared pages in the system: set run 2 to
	unmerge pages first, then to 1 after changing		unmerge pages first, then to 1 after changing
	merge_across_nodes, to remerge according to the new setting.		``merge_across_nodes``, to remerge according to the new setting.

	Default: 1 (merging across nodes as in earlier releases)		Default: 1 (merging across nodes as in earlier releases)

	run		run
	set 0 to stop ksmd from running but keep merged pages,		* set to 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 to 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		* set to 2 to stop ksmd and unmerge all pages currently merged, but
	leave mergeable areas registered for next run Default: 0 (must		leave mergeable areas registered for next run.
	be changed to 1 to activate KSM, except if CONFIG_SYSFS is
	disabled)		Default: 0 (must be changed to 1 to activate KSM, except if
			CONFIG_SYSFS is disabled)

	use_zero_pages		use_zero_pages
	specifies whether empty pages (i.e. allocated pages that only		specifies whether empty pages (i.e. allocated pages that only
	@@ -102,8 +127,9 @@ use_zero_pages
	KSM for some workloads, for example if the checksums of pages		KSM for some workloads, for example if the checksums of pages
	candidate for merging match the checksum of an empty		candidate for merging match the checksum of an empty
	page. This setting can be changed at any time, it is only		page. This setting can be changed at any time, it is only
	effective for pages merged after the change. Default: 0		effective for pages merged after the change.
	(normal KSM behaviour as in earlier releases)
			Default: 0 (normal KSM behaviour as in earlier releases)

	max_page_sharing		max_page_sharing
	Maximum sharing allowed for each KSM page. This enforces a		Maximum sharing allowed for each KSM page. This enforces a
	@@ -112,7 +138,7 @@ max_page_sharing
	page will have at least two sharers. The rmap walk has O(N)		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		complexity where N is the number of rmap_items (i.e. virtual
	mappings) that are sharing the page, which is in turn capped		mappings) that are sharing the page, which is in turn capped
	by max_page_sharing. So this effectively spread the the linear		by ``max_page_sharing``. So this effectively spreads the linear
	O(N) computational complexity from rmap walk context over		O(N) computational complexity from rmap walk context over
	different KSM pages. The ksmd walk over the stable_node		different KSM pages. The ksmd walk over the stable_node
	"chains" is also O(N), but N is the number of stable_node		"chains" is also O(N), but N is the number of stable_node
	@@ -140,7 +166,7 @@ stable_node_chains_prune_millisecs
	metadata with lower latency, but they will make ksmd use more		metadata with lower latency, but they will make ksmd use more
	CPU during the scan. This only applies to the stable_node		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		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		``max_page_sharing`` yet (there would be no stable_node chains in
	such case).		such case).

	The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:		The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
	@@ -157,21 +183,22 @@ full_scans
	how many times all mergeable areas have been scanned		how many times all mergeable areas have been scanned
	stable_node_chains		stable_node_chains
	number of stable node chains allocated, this is effectively		number of stable node chains allocated, this is effectively
	the number of KSM pages that hit the max_page_sharing limit		the number of KSM pages that hit the ``max_page_sharing`` limit
	stable_node_dups		stable_node_dups
	number of stable node dups queued into the stable_node chains		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_sharing`` to ``pages_shared`` indicates good
	a high ratio of pages_unshared to pages_sharing indicates wasted effort.		sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
	pages_volatile embraces several different kinds of activity, but a high		indicates wasted effort. ``pages_volatile`` embraces several
	proportion there would also indicate poor use of madvise MADV_MERGEABLE.		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		The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
	max_page_sharing tunable. To increase the ratio max_page_sharing must		``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
	be increased accordingly.		be increased accordingly.

	The stable_node_dups/stable_node_chains ratio is also affected by the		The ``stable_node_dups/stable_node_chains`` ratio is also affected by the
	max_page_sharing tunable, and an high ratio may indicate fragmentation		``max_page_sharing`` tunable, and an high ratio may indicate fragmentation
	in the stable_node dups, which could be solved by introducing		in the stable_node dups, which could be solved by introducing
	fragmentation algorithms in ksmd which would refile rmap_items from		fragmentation algorithms in ksmd which would refile rmap_items from
	one stable_node dup to another stable_node dup, in order to free up		one stable_node dup to another stable_node dup, in order to free up
	@@ -179,5 +206,6 @@ 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 ksmd CPU usage and possibly slowdown the readonly computations on
	the KSM pages of the applications.		the KSM pages of the applications.

			--
	Izik Eidus,		Izik Eidus,
	Hugh Dickins, 17 Nov 2009		Hugh Dickins, 17 Nov 2009