How to use the Kernel Samepage Merging feature
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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.

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)

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
