--- /dev/null
+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)
+
+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)
+
+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
Code Review / kvmfornfv.git / blobdiff