242 lines
11 KiB
ReStructuredText
242 lines
11 KiB
ReStructuredText
=======================
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Kernel Samepage Merging
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=======================
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Overview
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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 https://lwn.net/Articles/330589/
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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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The KSM daemon ksmd periodically scans those areas of user memory
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which have been registered with it, looking for pages of identical
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content which can be replaced by a single write-protected page (which
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is automatically copied if a process later wants to update its
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content). The amount of pages that KSM daemon scans in a single pass
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and the time between the passes are configured using :ref:`sysfs
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intraface <ksm_sysfs>`
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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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Controlling KSM with madvise
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============================
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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::
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int madvise(addr, length, MADV_MERGEABLE)
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The app may call
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::
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int madvise(addr, length, MADV_UNMERGEABLE)
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to cancel that advice and restore unshared pages: whereupon KSM
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unmerges whatever it merged in that range. Note: this unmerging call
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may suddenly require more memory than is available - possibly failing
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with EAGAIN, but more 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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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/admin-guide/sysctl/vm.rst).
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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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.. _ksm_sysfs:
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KSM daemon sysfs interface
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==========================
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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
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how many 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
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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
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specifies if pages from different NUMA nodes can be merged.
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When set to 0, ksm merges only pages which physically reside
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in the memory area of same NUMA node. That brings lower
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latency to access of shared pages. Systems with more nodes, at
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significant NUMA distances, are likely to benefit from the
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lower latency of setting 0. Smaller systems, which need to
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minimize memory usage, are likely to benefit from the greater
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sharing of setting 1 (default). You may wish to compare how
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your system performs under each setting, before deciding on
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which to use. ``merge_across_nodes`` setting can be changed only
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when there are no ksm shared pages in the system: set run 2 to
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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
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* set to 0 to stop ksmd from running but keep merged pages,
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* set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
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* set to 2 to stop ksmd and unmerge all pages currently merged, but
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leave mergeable areas registered for next run.
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Default: 0 (must be changed to 1 to activate KSM, except if
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CONFIG_SYSFS is disabled)
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use_zero_pages
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specifies whether empty pages (i.e. allocated pages that only
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contain zeroes) should be treated specially. When set to 1,
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empty pages are merged with the kernel zero page(s) instead of
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with each other as it would happen normally. This can improve
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the performance on architectures with coloured zero pages,
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depending on the workload. Care should be taken when enabling
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this setting, as it can potentially degrade the performance of
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KSM for some workloads, for example if the checksums of pages
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candidate for merging match the checksum of an empty
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page. This setting can be changed at any time, it is only
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effective for pages merged after the change.
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Default: 0 (normal KSM behaviour as in earlier releases)
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max_page_sharing
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Maximum sharing allowed for each KSM page. This enforces a
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deduplication limit to avoid high latency for virtual memory
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operations that involve traversal of the virtual mappings that
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share the KSM page. The minimum value is 2 as a newly created
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KSM page will have at least two sharers. The higher this value
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the faster KSM will merge the memory and the higher the
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deduplication factor will be, but the slower the worst case
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virtual mappings traversal could be for any given KSM
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page. Slowing down this traversal means there will be higher
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latency for certain virtual memory operations happening during
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swapping, compaction, NUMA balancing and page migration, in
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turn decreasing responsiveness for the caller of those virtual
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memory operations. The scheduler latency of other tasks not
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involved with the VM operations doing the virtual mappings
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traversal is not affected by this parameter as these
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traversals are always schedule friendly themselves.
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stable_node_chains_prune_millisecs
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specifies how frequently KSM checks the metadata of the pages
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that hit the deduplication limit for stale information.
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Smaller milllisecs values will free up the KSM metadata with
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lower latency, but they will make ksmd use more CPU during the
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scan. It's a noop if not a single KSM page hit the
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``max_page_sharing`` yet.
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The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
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pages_shared
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how many shared pages are being used
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pages_sharing
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how many more sites are sharing them i.e. how much saved
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pages_unshared
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how many pages unique but repeatedly checked for merging
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pages_volatile
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how many pages changing too fast to be placed in a tree
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full_scans
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how many times all mergeable areas have been scanned
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stable_node_chains
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the number of KSM pages that hit the ``max_page_sharing`` limit
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stable_node_dups
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number of duplicated KSM pages
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A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
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sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
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indicates wasted effort. ``pages_volatile`` embraces several
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different kinds of activity, but a high proportion there would also
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indicate poor use of madvise MADV_MERGEABLE.
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The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
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``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
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be increased accordingly.
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Monitoring KSM profit
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=====================
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KSM can save memory by merging identical pages, but also can consume
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additional memory, because it needs to generate a number of rmap_items to
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save each scanned page's brief rmap information. Some of these pages may
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be merged, but some may not be abled to be merged after being checked
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several times, which are unprofitable memory consumed.
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1) How to determine whether KSM save memory or consume memory in system-wide
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range? Here is a simple approximate calculation for reference::
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general_profit =~ pages_sharing * sizeof(page) - (all_rmap_items) *
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sizeof(rmap_item);
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where all_rmap_items can be easily obtained by summing ``pages_sharing``,
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``pages_shared``, ``pages_unshared`` and ``pages_volatile``.
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2) The KSM profit inner a single process can be similarly obtained by the
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following approximate calculation::
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process_profit =~ ksm_merging_pages * sizeof(page) -
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ksm_rmap_items * sizeof(rmap_item).
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where ksm_merging_pages is shown under the directory ``/proc/<pid>/``,
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and ksm_rmap_items is shown in ``/proc/<pid>/ksm_stat``.
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From the perspective of application, a high ratio of ``ksm_rmap_items`` to
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``ksm_merging_pages`` means a bad madvise-applied policy, so developers or
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administrators have to rethink how to change madvise policy. Giving an example
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for reference, a page's size is usually 4K, and the rmap_item's size is
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separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
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so if the ``ksm_rmap_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU
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or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
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because the ksm profit is approximately zero or negative.
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Monitoring KSM events
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=====================
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There are some counters in /proc/vmstat that may be used to monitor KSM events.
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KSM might help save memory, it's a tradeoff by may suffering delay on KSM COW
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or on swapping in copy. Those events could help users evaluate whether or how
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to use KSM. For example, if cow_ksm increases too fast, user may decrease the
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range of madvise(, , MADV_MERGEABLE).
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cow_ksm
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is incremented every time a KSM page triggers copy on write (COW)
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when users try to write to a KSM page, we have to make a copy.
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ksm_swpin_copy
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is incremented every time a KSM page is copied when swapping in
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note that KSM page might be copied when swapping in because do_swap_page()
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cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.
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--
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Izik Eidus,
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Hugh Dickins, 17 Nov 2009
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