linux-next/Documentation/vm/ksm.rst

.. _ksm:

=======================
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 http://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
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.

.. _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.

Design
======

Overview
--------

.. kernel-doc:: mm/ksm.c
   :DOC: Overview

Reverse mapping
---------------
KSM maintains reverse mapping information for KSM pages in the stable
tree.

If a KSM page is shared between less than ``max_page_sharing`` VMAs,
the node of the stable tree that represents such KSM page points to a
list of :c:type:`struct rmap_item` and the ``page->mapping`` of the
KSM page points to the stable tree node.

When the sharing passes this threshold, KSM adds a second dimension to
the stable tree. The tree node becomes a "chain" that links one or
more "dups". Each "dup" keeps reverse mapping information for a KSM
page with ``page->mapping`` pointing to that "dup".

Every "chain" and all "dups" linked into a "chain" enforce the
invariant that they represent the same write protected memory content,
even if each "dup" will be pointed by a different KSM page copy of
that content.

This way the stable tree lookup computational complexity is unaffected
if compared to an unlimited list of reverse mappings. It is still
enforced that there cannot be KSM page content duplicates in the
stable tree itself.

The deduplication limit enforced by ``max_page_sharing`` is required
to avoid the virtual memory rmap lists to grow too large. 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 spreads 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.

High values of ``max_page_sharing`` result in faster memory merging
(because there will be fewer stable_node dups queued into the
stable_node chain->hlist to check for pruning) and higher
deduplication factor at the expense of slower worst case for rmap
walks for any KSM page which can happen during swapping, compaction,
NUMA balancing and page migration.

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 free up
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.

The whole list of stable_node "dups" linked in the stable_node
"chains" is scanned periodically in order to prune stale stable_nodes.
The frequency of such scans is defined by
``stable_node_chains_prune_millisecs`` sysfs tunable.

Reference
---------
.. kernel-doc:: mm/ksm.c
   :functions: mm_slot ksm_scan stable_node rmap_item

--
Izik Eidus,
Hugh Dickins, 17 Nov 2009