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diff --git a/Documentation/vm/ksm.txt b/Documentation/vm/ksm.txt deleted file mode 100644 index b392e496f81..00000000000 --- a/Documentation/vm/ksm.txt +++ /dev/null @@ -1,82 +0,0 @@ -How to use the Kernel Samepage Merging feature ----------------------------------------------- - -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 http://lwn.net/Articles/330589/ - -The KSM daemon ksmd periodically scans those areas of user memory which -have been registered with it, looking for pages of identical content which -can be replaced by a single write-protected page (which is automatically -copied if a process later wants to update its content). - -KSM was originally developed for use with KVM (where it was known as -Kernel Shared Memory), to fit more virtual machines into physical memory, -by sharing the data common between them. But it can be useful to any -application which generates many instances of the same data. - -KSM only merges anonymous (private) pages, never pagecache (file) pages. -KSM's merged pages were originally locked into kernel memory, but can now -be swapped out just like other user pages (but sharing is broken when they -are swapped back in: ksmd must rediscover their identity and merge again). - -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. - -If KSM is not configured into the running kernel, madvise MADV_MERGEABLE -and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was -built with CONFIG_KSM=y, those calls will normally succeed: even if the -the KSM daemon is not currently running, MADV_MERGEABLE still registers -the range for whenever the KSM daemon is started; even if the range -cannot contain any pages which KSM could actually merge; even if -MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE. - -Like other madvise calls, they are intended for use on mapped areas of -the user address space: they will report ENOMEM if the specified range -includes unmapped gaps (though working on the intervening mapped areas), -and might fail with EAGAIN if not enough memory for internal structures. - -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/, -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) - -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) - -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 - -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. -pages_volatile embraces several different kinds of activity, but a high -proportion there would also indicate poor use of madvise MADV_MERGEABLE. - -Izik Eidus, -Hugh Dickins, 17 Nov 2009 |