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+.. _admin_guide_ksm:
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+
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+=======================
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+Kernel Samepage Merging
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+=======================
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+
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+Overview
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+========
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+
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+KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
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+added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation,
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+and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/
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+
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+KSM was originally developed for use with KVM (where it was known as
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+Kernel Shared Memory), to fit more virtual machines into physical memory,
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+by sharing the data common between them. But it can be useful to any
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+application which generates many instances of the same data.
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+
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+The KSM daemon ksmd periodically scans those areas of user memory
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+which have been registered with it, looking for pages of identical
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+content which can be replaced by a single write-protected page (which
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+is automatically copied if a process later wants to update its
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+content). The amount of pages that KSM daemon scans in a single pass
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+and the time between the passes are configured using :ref:`sysfs
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+intraface <ksm_sysfs>`
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+
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+KSM only merges anonymous (private) pages, never pagecache (file) pages.
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+KSM's merged pages were originally locked into kernel memory, but can now
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+be swapped out just like other user pages (but sharing is broken when they
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+are swapped back in: ksmd must rediscover their identity and merge again).
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+
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+Controlling KSM with madvise
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+============================
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+
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+KSM only operates on those areas of address space which an application
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+has advised to be likely candidates for merging, by using the madvise(2)
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+system call::
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+
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+ int madvise(addr, length, MADV_MERGEABLE)
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+
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+The app may call
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+
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+::
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+
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+ int madvise(addr, length, MADV_UNMERGEABLE)
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+
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+to cancel that advice and restore unshared pages: whereupon KSM
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+unmerges whatever it merged in that range. Note: this unmerging call
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+may suddenly require more memory than is available - possibly failing
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+with EAGAIN, but more probably arousing the Out-Of-Memory killer.
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+
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+If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
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+and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
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+built with CONFIG_KSM=y, those calls will normally succeed: even if the
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+the KSM daemon is not currently running, MADV_MERGEABLE still registers
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+the range for whenever the KSM daemon is started; even if the range
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+cannot contain any pages which KSM could actually merge; even if
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+MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
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+
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+If a region of memory must be split into at least one new MADV_MERGEABLE
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+or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
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+will exceed ``vm.max_map_count`` (see Documentation/sysctl/vm.txt).
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+
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+Like other madvise calls, they are intended for use on mapped areas of
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+the user address space: they will report ENOMEM if the specified range
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+includes unmapped gaps (though working on the intervening mapped areas),
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+and might fail with EAGAIN if not enough memory for internal structures.
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+
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+Applications should be considerate in their use of MADV_MERGEABLE,
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+restricting its use to areas likely to benefit. KSM's scans may use a lot
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+of processing power: some installations will disable KSM for that reason.
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+
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+.. _ksm_sysfs:
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+
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+KSM daemon sysfs interface
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+==========================
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+
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+The KSM daemon is controlled by sysfs files in ``/sys/kernel/mm/ksm/``,
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+readable by all but writable only by root:
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+
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+pages_to_scan
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+ how many pages to scan before ksmd goes to sleep
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+ e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
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+
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+ Default: 100 (chosen for demonstration purposes)
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+
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+sleep_millisecs
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+ how many milliseconds ksmd should sleep before next scan
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+ e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs``
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+
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+ Default: 20 (chosen for demonstration purposes)
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+
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+merge_across_nodes
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+ specifies if pages from different NUMA nodes can be merged.
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+ When set to 0, ksm merges only pages which physically reside
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+ in the memory area of same NUMA node. That brings lower
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+ latency to access of shared pages. Systems with more nodes, at
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+ significant NUMA distances, are likely to benefit from the
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+ lower latency of setting 0. Smaller systems, which need to
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+ minimize memory usage, are likely to benefit from the greater
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+ sharing of setting 1 (default). You may wish to compare how
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+ your system performs under each setting, before deciding on
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+ which to use. ``merge_across_nodes`` setting can be changed only
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+ when there are no ksm shared pages in the system: set run 2 to
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+ unmerge pages first, then to 1 after changing
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+ ``merge_across_nodes``, to remerge according to the new setting.
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+
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+ Default: 1 (merging across nodes as in earlier releases)
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+
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+run
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+ * set to 0 to stop ksmd from running but keep merged pages,
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+ * set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
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+ * set to 2 to stop ksmd and unmerge all pages currently merged, but
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+ leave mergeable areas registered for next run.
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+
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+ Default: 0 (must be changed to 1 to activate KSM, except if
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+ CONFIG_SYSFS is disabled)
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+
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+use_zero_pages
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+ specifies whether empty pages (i.e. allocated pages that only
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+ contain zeroes) should be treated specially. When set to 1,
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+ empty pages are merged with the kernel zero page(s) instead of
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+ with each other as it would happen normally. This can improve
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+ the performance on architectures with coloured zero pages,
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+ depending on the workload. Care should be taken when enabling
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+ this setting, as it can potentially degrade the performance of
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+ KSM for some workloads, for example if the checksums of pages
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+ candidate for merging match the checksum of an empty
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+ page. This setting can be changed at any time, it is only
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+ effective for pages merged after the change.
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+
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+ Default: 0 (normal KSM behaviour as in earlier releases)
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+
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+max_page_sharing
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+ Maximum sharing allowed for each KSM page. This enforces a
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+ deduplication limit to avoid high latency for virtual memory
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+ operations that involve traversal of the virtual mappings that
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+ share the KSM page. The minimum value is 2 as a newly created
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+ KSM page will have at least two sharers. The higher this value
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+ the faster KSM will merge the memory and the higher the
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+ deduplication factor will be, but the slower the worst case
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+ virtual mappings traversal could be for any given KSM
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+ page. Slowing down this traversal means there will be higher
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+ latency for certain virtual memory operations happening during
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+ swapping, compaction, NUMA balancing and page migration, in
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+ turn decreasing responsiveness for the caller of those virtual
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+ memory operations. The scheduler latency of other tasks not
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+ involved with the VM operations doing the virtual mappings
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+ traversal is not affected by this parameter as these
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+ traversals are always schedule friendly themselves.
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+
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+stable_node_chains_prune_millisecs
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+ specifies how frequently KSM checks the metadata of the pages
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+ that hit the deduplication limit for stale information.
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+ Smaller milllisecs values will free up the KSM metadata with
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+ lower latency, but they will make ksmd use more CPU during the
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+ scan. It's a noop if not a single KSM page hit the
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+ ``max_page_sharing`` yet.
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+
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+The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
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+
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+pages_shared
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+ how many shared pages are being used
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+pages_sharing
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+ how many more sites are sharing them i.e. how much saved
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+pages_unshared
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+ how many pages unique but repeatedly checked for merging
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+pages_volatile
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+ how many pages changing too fast to be placed in a tree
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+full_scans
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+ how many times all mergeable areas have been scanned
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+stable_node_chains
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+ the number of KSM pages that hit the ``max_page_sharing`` limit
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+stable_node_dups
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+ number of duplicated KSM pages
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+
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+A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
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+sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
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+indicates wasted effort. ``pages_volatile`` embraces several
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+different kinds of activity, but a high proportion there would also
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+indicate poor use of madvise MADV_MERGEABLE.
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+
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+The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
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+``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
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+be increased accordingly.
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+
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+--
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+Izik Eidus,
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+Hugh Dickins, 17 Nov 2009
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Mike Rapoport