88 строки
3.4 KiB
ReStructuredText
88 строки
3.4 KiB
ReStructuredText
.. _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 https://lwn.net/Articles/330589/
|
|
|
|
The userspace interface of KSM is described in :ref:`Documentation/admin-guide/mm/ksm.rst <admin_guide_ksm>`
|
|
|
|
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 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
|