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Commit 2fcbc413 authored by Mike Rapoport's avatar Mike Rapoport Committed by Jonathan Corbet
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docs/vm: ksm.txt: convert to ReST format 

Signed-off-by: default avatarMike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
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Documentation/vm/ksm.txt
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How to use the Kernel Samepage Merging feature
----------------------------------------------
.. _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,
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/
The KSM daemon ksmd periodically scans those areas of user memory which
......@@ -51,110 +54,112 @@ 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.
The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
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 present 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 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 0 to stop ksmd from running but keep merged pages,
set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
set 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 the virtual
memory rmap lists to grow too large. The minimum
value is 2 as a newly created KSM page will have at
least two sharers. 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 spread the 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. The higher this value the
faster KSM will merge the memory (because there
will be fewer stable_node dups queued into the
stable_node chain->hlist to check for pruning) and
the higher the deduplication factor will be, but
the slowest the worst case rmap walk could be for
any given KSM page. Slowing down the rmap_walk
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 rmap walk is not
affected by this parameter as the rmap walks are
always schedule friendly themselves.
stable_node_chains_prune_millisecs - How frequently to walk the whole
list of stable_node "dups" linked in the
stable_node "chains" in order to prune stale
stable_nodes. Smaller milllisecs values will free
up the KSM metadata with lower latency, but they
will make ksmd use more CPU during the scan. This
only applies to the stable_node chains so it's a
noop if not a single KSM page hit the
max_page_sharing yet (there would be no stable_node
chains in such case).
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 - number of stable node chains allocated, this is
effectively the number of KSM pages that hit the
max_page_sharing limit
stable_node_dups - number of stable node dups queued into the
stable_node chains
pages_to_scan
how many present 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 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 0 to stop ksmd from running but keep merged pages,
set 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
set 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 the virtual memory rmap lists to
grow too large. The minimum value is 2 as a newly created KSM
page will have at least two sharers. 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 spread the 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. The higher this value the faster KSM will
merge the memory (because there will be fewer stable_node dups
queued into the stable_node chain->hlist to check for pruning)
and the higher the deduplication factor will be, but the
slowest the worst case rmap walk could be for any given KSM
page. Slowing down the rmap_walk 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 rmap walk is not
affected by this parameter as the rmap walks are always
schedule friendly themselves.
stable_node_chains_prune_millisecs
How frequently to walk the whole list of stable_node "dups"
linked in the stable_node "chains" in order to prune stale
stable_nodes. Smaller milllisecs values will free up the KSM
metadata with lower latency, but they will make ksmd use more
CPU during the scan. This only applies to the stable_node
chains so it's a noop if not a single KSM page hit the
max_page_sharing yet (there would be no stable_node chains in
such case).
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
number of stable node chains allocated, this is effectively
the number of KSM pages that hit the max_page_sharing limit
stable_node_dups
number of stable node dups queued into the stable_node chains
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.
......
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