116 lines
6.5 KiB
Plaintext
116 lines
6.5 KiB
Plaintext
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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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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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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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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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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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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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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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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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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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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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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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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Default: 0 (must be changed to 1 to activate KSM,
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except if CONFIG_SYSFS is disabled)
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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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The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
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pages_shared - how many shared pages are being used
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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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Hugh Dickins, 17 Nov 2009
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