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+.. _ksm:
+
+=======================
+Kernel Samepage Merging
+=======================
+
+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/
+
+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
+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.
+
+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).
+
+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.
+
+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 present 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 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 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 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 the virtual memory rmap lists to
+ grow too large. The minimum value is 2 as a newly created KSM
+ 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
+ mappings) that are sharing the page, which is in turn capped
+ by max_page_sharing. So this effectively spread the the linear
+ O(N) computational complexity from rmap walk context over
+ different KSM pages. The ksmd walk over the stable_node
+ "chains" is also O(N), but N is the number of stable_node
+ "dups", not the number of rmap_items, so it has not a
+ significant impact on ksmd performance. In practice the best
+ stable_node "dup" candidate will be kept and found at the head
+ of the "dups" list. The higher this value the faster KSM will
+ merge the memory (because there will be fewer stable_node dups
+ queued into the stable_node chain->hlist to check for pruning)
+ and the higher the deduplication factor will be, but the
+ slowest the worst case rmap walk could be for any given KSM
+ page. Slowing down the rmap_walk 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 rmap walk is not
+ affected by this parameter as the rmap walks are always
+ schedule friendly themselves.
+
+stable_node_chains_prune_millisecs
+ How frequently to walk the whole list of stable_node "dups"
+ linked in the stable_node "chains" in order to prune stale
+ stable_nodes. Smaller milllisecs values will free up the KSM
+ metadata with lower latency, but they will make ksmd use more
+ 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
+ max_page_sharing yet (there would be no stable_node chains in
+ such case).
+
+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
+ number of stable node chains allocated, this is effectively
+ the number of KSM pages that hit the max_page_sharing limit
+stable_node_dups
+ 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_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 page_sharing/page_shared ratio is limited by the
+max_page_sharing tunable. To increase the ratio max_page_sharing must
+be increased accordingly.
+
+The stable_node_dups/stable_node_chains ratio is also affected by the
+max_page_sharing tunable, and an high ratio may indicate fragmentation
+in the stable_node dups, which could be solved by introducing
+fragmentation algorithms in ksmd which would refile rmap_items from
+one stable_node dup to another stable_node dup, in order to freeup
+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 KSM pages of the applications.
+
+Izik Eidus,
+Hugh Dickins, 17 Nov 2009