=======================
Kernel Samepage Merging
=======================

Overview
========

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 https://lwn.net/Articles/330589/

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.

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). The amount of pages that KSM daemon scans in a single pass
and the time between the passes are configured using :ref:`sysfs
intraface <ksm_sysfs>`

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).

Controlling KSM with madvise
============================

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
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/admin-guide/sysctl/vm.rst).

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.

.. _ksm_sysfs:

KSM daemon sysfs interface
==========================

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 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 the 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 to 0 to stop ksmd from running but keep merged pages,
        * set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
        * set to 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 high latency for virtual memory
        operations that involve traversal of the virtual mappings that
        share the KSM page. The minimum value is 2 as a newly created
        KSM page will have at least two sharers. The higher this value
        the faster KSM will merge the memory and the higher the
        deduplication factor will be, but the slower the worst case
        virtual mappings traversal could be for any given KSM
        page. Slowing down this traversal 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 virtual mappings
        traversal is not affected by this parameter as these
        traversals are always schedule friendly themselves.

stable_node_chains_prune_millisecs
        specifies how frequently KSM checks the metadata of the pages
        that hit the deduplication limit for stale information.
        Smaller milllisecs values will free up the KSM metadata with
        lower latency, but they will make ksmd use more CPU during the
        scan. It's a noop if not a single KSM page hit the
        ``max_page_sharing`` yet.

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
        the number of KSM pages that hit the ``max_page_sharing`` limit
stable_node_dups
        number of duplicated KSM pages

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 ``pages_sharing/pages_shared`` ratio is limited by the
``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
be increased accordingly.

Monitoring KSM profit
=====================

KSM can save memory by merging identical pages, but also can consume
additional memory, because it needs to generate a number of rmap_items to
save each scanned page's brief rmap information. Some of these pages may
be merged, but some may not be abled to be merged after being checked
several times, which are unprofitable memory consumed.

1) How to determine whether KSM save memory or consume memory in system-wide
   range? Here is a simple approximate calculation for reference::

	general_profit =~ pages_sharing * sizeof(page) - (all_rmap_items) *
			  sizeof(rmap_item);

   where all_rmap_items can be easily obtained by summing ``pages_sharing``,
   ``pages_shared``, ``pages_unshared`` and ``pages_volatile``.

2) The KSM profit inner a single process can be similarly obtained by the
   following approximate calculation::

	process_profit =~ ksm_merging_pages * sizeof(page) -
			  ksm_rmap_items * sizeof(rmap_item).

   where ksm_merging_pages is shown under the directory ``/proc/<pid>/``,
   and ksm_rmap_items is shown in ``/proc/<pid>/ksm_stat``.

From the perspective of application, a high ratio of ``ksm_rmap_items`` to
``ksm_merging_pages`` means a bad madvise-applied policy, so developers or
administrators have to rethink how to change madvise policy. Giving an example
for reference, a page's size is usually 4K, and the rmap_item's size is
separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
so if the ``ksm_rmap_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU
or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
because the ksm profit is approximately zero or negative.

Monitoring KSM events
=====================

There are some counters in /proc/vmstat that may be used to monitor KSM events.
KSM might help save memory, it's a tradeoff by may suffering delay on KSM COW
or on swapping in copy. Those events could help users evaluate whether or how
to use KSM. For example, if cow_ksm increases too fast, user may decrease the
range of madvise(, , MADV_MERGEABLE).

cow_ksm
	is incremented every time a KSM page triggers copy on write (COW)
	when users try to write to a KSM page, we have to make a copy.

ksm_swpin_copy
	is incremented every time a KSM page is copied when swapping in
	note that KSM page might be copied when swapping in because do_swap_page()
	cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.

--
Izik Eidus,
Hugh Dickins, 17 Nov 2009