linux/Documentation/vm/ksm.rst

.. _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