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-How to use the Kernel Samepage Merging feature
-----------------------------------------------
-
-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