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88 lines
3.4 KiB
ReStructuredText
.. _ksm:
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=======================
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Kernel Samepage Merging
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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 https://lwn.net/Articles/330589/
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The userspace interface of KSM is described in :ref:`Documentation/admin-guide/mm/ksm.rst <admin_guide_ksm>`
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Design
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======
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Overview
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--------
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.. kernel-doc:: mm/ksm.c
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:DOC: Overview
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Reverse mapping
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---------------
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KSM maintains reverse mapping information for KSM pages in the stable
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tree.
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If a KSM page is shared between less than ``max_page_sharing`` VMAs,
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the node of the stable tree that represents such KSM page points to a
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list of struct rmap_item and the ``page->mapping`` of the
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KSM page points to the stable tree node.
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When the sharing passes this threshold, KSM adds a second dimension to
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the stable tree. The tree node becomes a "chain" that links one or
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more "dups". Each "dup" keeps reverse mapping information for a KSM
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page with ``page->mapping`` pointing to that "dup".
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Every "chain" and all "dups" linked into a "chain" enforce the
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invariant that they represent the same write protected memory content,
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even if each "dup" will be pointed by a different KSM page copy of
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that content.
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This way the stable tree lookup computational complexity is unaffected
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if compared to an unlimited list of reverse mappings. It is still
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enforced that there cannot be KSM page content duplicates in the
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stable tree itself.
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The deduplication limit enforced by ``max_page_sharing`` is required
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to avoid the virtual memory rmap lists to grow too large. The rmap
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walk has O(N) complexity where N is the number of rmap_items
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(i.e. virtual mappings) that are sharing the page, which is in turn
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capped by ``max_page_sharing``. So this effectively spreads the linear
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O(N) computational complexity from rmap walk context over different
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KSM pages. The ksmd walk over the stable_node "chains" is also O(N),
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but N is the number of stable_node "dups", not the number of
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rmap_items, so it has not a significant impact on ksmd performance. In
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practice the best stable_node "dup" candidate will be kept and found
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at the head of the "dups" list.
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High values of ``max_page_sharing`` result in faster memory merging
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(because there will be fewer stable_node dups queued into the
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stable_node chain->hlist to check for pruning) and higher
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deduplication factor at the expense of slower worst case for rmap
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walks for any KSM page which can happen during swapping, compaction,
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NUMA balancing and page migration.
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The ``stable_node_dups/stable_node_chains`` ratio is also affected by the
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``max_page_sharing`` tunable, and an high ratio may indicate fragmentation
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in the stable_node dups, which could be solved by introducing
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fragmentation algorithms in ksmd which would refile rmap_items from
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one stable_node dup to another stable_node dup, in order to free up
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stable_node "dups" with few rmap_items in them, but that may increase
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the ksmd CPU usage and possibly slowdown the readonly computations on
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the KSM pages of the applications.
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The whole list of stable_node "dups" linked in the stable_node
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"chains" is scanned periodically in order to prune stale stable_nodes.
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The frequency of such scans is defined by
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``stable_node_chains_prune_millisecs`` sysfs tunable.
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Reference
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---------
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.. kernel-doc:: mm/ksm.c
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:functions: mm_slot ksm_scan stable_node rmap_item
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--
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Izik Eidus,
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Hugh Dickins, 17 Nov 2009
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