KSM - Revision history

Speculatrix: /* Kernel Samepage Merging */ reworded original cumbersome wording

2015-07-09T09:53:30Z

Kernel Samepage Merging: reworded original cumbersome wording

← Older revision		Revision as of 05:53, 9 July 2015
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	= Kernel Samepage Merging =		= Kernel Samepage Merging =

	KSM is a memory-saving de-duplication feature, that merges anonymous (private) pages (not pagecache ones). Although it started this way, KSM is currently ~~not~~ suitable ~~only~~ for Virtual Machine use, ~~but rather,~~ it can be useful to any		KSM is a memory-saving de-duplication feature, that merges anonymous (private) pages (not pagecache ones). Although it started this way, KSM is currently suitable for more than Virtual Machine use, as it can be useful to any application which generates many instances of the same data
	application which generates many instances of the same data

	== Enabling KSM ==		== Enabling KSM ==

Bsd: Add categories

2015-05-16T21:03:10Z

Add categories

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			[[Category:Architecture]][[Category:Docs]][[Category:HowTo]]

Glommer at 13:22, 19 January 2010

2010-01-19T13:22:47Z

New page

= Kernel Samepage Merging =

KSM is a memory-saving de-duplication feature, that merges anonymous (private) pages (not pagecache ones). Although it started this way, KSM is currently not suitable only for Virtual Machine use, but rather, it can be useful to any
application which generates many instances of the same data

== Enabling KSM ==

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.

== Status ==

Merged in linux 2.6.32. One needs to enable it via CONFIG_KSM=y

== Results ==

TBD

== Testing ==

You can verify KSM is in action, by checking for the existence of some of its /sys files, under /sys/kernel/mm/ksm/

They are:

{|
|-
|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
|-
|}