2009-09-22 07:02:24 +07:00
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How to use the Kernel Samepage Merging feature
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----------------------------------------------
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KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
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added to the Linux kernel in 2.6.32. See mm/ksm.c for its implementation,
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and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/
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The KSM daemon ksmd periodically scans those areas of user memory which
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have been registered with it, looking for pages of identical content which
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can be replaced by a single write-protected page (which is automatically
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copied if a process later wants to update its content).
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KSM was originally developed for use with KVM (where it was known as
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Kernel Shared Memory), to fit more virtual machines into physical memory,
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by sharing the data common between them. But it can be useful to any
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application which generates many instances of the same data.
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KSM only merges anonymous (private) pages, never pagecache (file) pages.
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2009-12-15 08:59:34 +07:00
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KSM's merged pages were originally locked into kernel memory, but can now
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be swapped out just like other user pages (but sharing is broken when they
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are swapped back in: ksmd must rediscover their identity and merge again).
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2009-09-22 07:02:24 +07:00
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KSM only operates on those areas of address space which an application
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has advised to be likely candidates for merging, by using the madvise(2)
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system call: int madvise(addr, length, MADV_MERGEABLE).
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The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel
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that advice and restore unshared pages: whereupon KSM unmerges whatever
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it merged in that range. Note: this unmerging call may suddenly require
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more memory than is available - possibly failing with EAGAIN, but more
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probably arousing the Out-Of-Memory killer.
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If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
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and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
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built with CONFIG_KSM=y, those calls will normally succeed: even if the
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the KSM daemon is not currently running, MADV_MERGEABLE still registers
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the range for whenever the KSM daemon is started; even if the range
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cannot contain any pages which KSM could actually merge; even if
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MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
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2017-02-25 05:58:47 +07:00
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If a region of memory must be split into at least one new MADV_MERGEABLE
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or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
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will exceed vm.max_map_count (see Documentation/sysctl/vm.txt).
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2009-09-22 07:02:24 +07:00
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Like other madvise calls, they are intended for use on mapped areas of
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the user address space: they will report ENOMEM if the specified range
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includes unmapped gaps (though working on the intervening mapped areas),
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and might fail with EAGAIN if not enough memory for internal structures.
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Applications should be considerate in their use of MADV_MERGEABLE,
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2009-12-15 08:59:34 +07:00
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restricting its use to areas likely to benefit. KSM's scans may use a lot
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of processing power: some installations will disable KSM for that reason.
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2009-09-22 07:02:24 +07:00
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The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
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readable by all but writable only by root:
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pages_to_scan - how many present pages to scan before ksmd goes to sleep
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2009-10-08 06:32:22 +07:00
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e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan"
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Default: 100 (chosen for demonstration purposes)
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2009-09-22 07:02:24 +07:00
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sleep_millisecs - how many milliseconds ksmd should sleep before next scan
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e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs"
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Default: 20 (chosen for demonstration purposes)
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2013-02-23 07:35:00 +07:00
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merge_across_nodes - specifies if pages from different numa nodes can be merged.
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When set to 0, ksm merges only pages which physically
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2013-02-23 07:36:03 +07:00
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reside in the memory area of same NUMA node. That brings
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lower latency to access of shared pages. Systems with more
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nodes, at significant NUMA distances, are likely to benefit
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from the lower latency of setting 0. Smaller systems, which
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need to minimize memory usage, are likely to benefit from
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the greater sharing of setting 1 (default). You may wish to
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compare how your system performs under each setting, before
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deciding on which to use. merge_across_nodes setting can be
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changed only when there are no ksm shared pages in system:
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set run 2 to unmerge pages first, then to 1 after changing
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merge_across_nodes, to remerge according to the new setting.
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Default: 1 (merging across nodes as in earlier releases)
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2013-02-23 07:35:00 +07:00
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2009-09-22 07:02:24 +07:00
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run - set 0 to stop ksmd from running but keep merged pages,
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set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
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set 2 to stop ksmd and unmerge all pages currently merged,
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but leave mergeable areas registered for next run
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2009-10-08 06:32:22 +07:00
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Default: 0 (must be changed to 1 to activate KSM,
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except if CONFIG_SYSFS is disabled)
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2009-09-22 07:02:24 +07:00
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mm/ksm: improve deduplication of zero pages with colouring
Some architectures have a set of zero pages (coloured zero pages)
instead of only one zero page, in order to improve the cache
performance. In those cases, the kernel samepage merger (KSM) would
merge all the allocated pages that happen to be filled with zeroes to
the same deduplicated page, thus losing all the advantages of coloured
zero pages.
This behaviour is noticeable when a process accesses large arrays of
allocated pages containing zeroes. A test I conducted on s390 shows
that there is a speed penalty when KSM merges such pages, compared to
not merging them or using actual zero pages from the start without
breaking the COW.
This patch fixes this behaviour. When coloured zero pages are present,
the checksum of a zero page is calculated during initialisation, and
compared with the checksum of the current canditate during merging. In
case of a match, the normal merging routine is used to merge the page
with the correct coloured zero page, which ensures the candidate page is
checked to be equal to the target zero page.
A sysfs entry is also added to toggle this behaviour, since it can
potentially introduce performance regressions, especially on
architectures without coloured zero pages. The default value is
disabled, for backwards compatibility.
With this patch, the performance with KSM is the same as with non
COW-broken actual zero pages, which is also the same as without KSM.
[akpm@linux-foundation.org: make zero_checksum and ksm_use_zero_pages __read_mostly, per Andrea]
[imbrenda@linux.vnet.ibm.com: documentation for coloured zero pages deduplication]
Link: http://lkml.kernel.org/r/1484927522-1964-1-git-send-email-imbrenda@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1484850953-23941-1-git-send-email-imbrenda@linux.vnet.ibm.com
Signed-off-by: Claudio Imbrenda <imbrenda@linux.vnet.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2017-02-25 05:55:39 +07:00
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use_zero_pages - specifies whether empty pages (i.e. allocated pages
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that only contain zeroes) should be treated specially.
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When set to 1, empty pages are merged with the kernel
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zero page(s) instead of with each other as it would
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happen normally. This can improve the performance on
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architectures with coloured zero pages, depending on
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the workload. Care should be taken when enabling this
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setting, as it can potentially degrade the performance
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of KSM for some workloads, for example if the checksums
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of pages candidate for merging match the checksum of
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an empty page. This setting can be changed at any time,
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it is only effective for pages merged after the change.
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Default: 0 (normal KSM behaviour as in earlier releases)
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2009-09-22 07:02:24 +07:00
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The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
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2009-12-15 08:59:34 +07:00
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pages_shared - how many shared pages are being used
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2009-09-22 07:02:24 +07:00
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pages_sharing - how many more sites are sharing them i.e. how much saved
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pages_unshared - how many pages unique but repeatedly checked for merging
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pages_volatile - how many pages changing too fast to be placed in a tree
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full_scans - how many times all mergeable areas have been scanned
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A high ratio of pages_sharing to pages_shared indicates good sharing, but
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a high ratio of pages_unshared to pages_sharing indicates wasted effort.
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pages_volatile embraces several different kinds of activity, but a high
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proportion there would also indicate poor use of madvise MADV_MERGEABLE.
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Izik Eidus,
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2009-12-15 08:59:34 +07:00
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Hugh Dickins, 17 Nov 2009
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