Based on kernel version 3.19. Page generated on 2015-02-13 21:23 EST.
1 How to use the Kernel Samepage Merging feature 2 ---------------------------------------------- 3 4 KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y, 5 added to the Linux kernel in 2.6.32. See mm/ksm.c for its implementation, 6 and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/ 7 8 The KSM daemon ksmd periodically scans those areas of user memory which 9 have been registered with it, looking for pages of identical content which 10 can be replaced by a single write-protected page (which is automatically 11 copied if a process later wants to update its content). 12 13 KSM was originally developed for use with KVM (where it was known as 14 Kernel Shared Memory), to fit more virtual machines into physical memory, 15 by sharing the data common between them. But it can be useful to any 16 application which generates many instances of the same data. 17 18 KSM only merges anonymous (private) pages, never pagecache (file) pages. 19 KSM's merged pages were originally locked into kernel memory, but can now 20 be swapped out just like other user pages (but sharing is broken when they 21 are swapped back in: ksmd must rediscover their identity and merge again). 22 23 KSM only operates on those areas of address space which an application 24 has advised to be likely candidates for merging, by using the madvise(2) 25 system call: int madvise(addr, length, MADV_MERGEABLE). 26 27 The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel 28 that advice and restore unshared pages: whereupon KSM unmerges whatever 29 it merged in that range. Note: this unmerging call may suddenly require 30 more memory than is available - possibly failing with EAGAIN, but more 31 probably arousing the Out-Of-Memory killer. 32 33 If KSM is not configured into the running kernel, madvise MADV_MERGEABLE 34 and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was 35 built with CONFIG_KSM=y, those calls will normally succeed: even if the 36 the KSM daemon is not currently running, MADV_MERGEABLE still registers 37 the range for whenever the KSM daemon is started; even if the range 38 cannot contain any pages which KSM could actually merge; even if 39 MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE. 40 41 Like other madvise calls, they are intended for use on mapped areas of 42 the user address space: they will report ENOMEM if the specified range 43 includes unmapped gaps (though working on the intervening mapped areas), 44 and might fail with EAGAIN if not enough memory for internal structures. 45 46 Applications should be considerate in their use of MADV_MERGEABLE, 47 restricting its use to areas likely to benefit. KSM's scans may use a lot 48 of processing power: some installations will disable KSM for that reason. 49 50 The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/, 51 readable by all but writable only by root: 52 53 pages_to_scan - how many present pages to scan before ksmd goes to sleep 54 e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan" 55 Default: 100 (chosen for demonstration purposes) 56 57 sleep_millisecs - how many milliseconds ksmd should sleep before next scan 58 e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs" 59 Default: 20 (chosen for demonstration purposes) 60 61 merge_across_nodes - specifies if pages from different numa nodes can be merged. 62 When set to 0, ksm merges only pages which physically 63 reside in the memory area of same NUMA node. That brings 64 lower latency to access of shared pages. Systems with more 65 nodes, at significant NUMA distances, are likely to benefit 66 from the lower latency of setting 0. Smaller systems, which 67 need to minimize memory usage, are likely to benefit from 68 the greater sharing of setting 1 (default). You may wish to 69 compare how your system performs under each setting, before 70 deciding on which to use. merge_across_nodes setting can be 71 changed only when there are no ksm shared pages in system: 72 set run 2 to unmerge pages first, then to 1 after changing 73 merge_across_nodes, to remerge according to the new setting. 74 Default: 1 (merging across nodes as in earlier releases) 75 76 run - set 0 to stop ksmd from running but keep merged pages, 77 set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run", 78 set 2 to stop ksmd and unmerge all pages currently merged, 79 but leave mergeable areas registered for next run 80 Default: 0 (must be changed to 1 to activate KSM, 81 except if CONFIG_SYSFS is disabled) 82 83 The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/: 84 85 pages_shared - how many shared pages are being used 86 pages_sharing - how many more sites are sharing them i.e. how much saved 87 pages_unshared - how many pages unique but repeatedly checked for merging 88 pages_volatile - how many pages changing too fast to be placed in a tree 89 full_scans - how many times all mergeable areas have been scanned 90 91 A high ratio of pages_sharing to pages_shared indicates good sharing, but 92 a high ratio of pages_unshared to pages_sharing indicates wasted effort. 93 pages_volatile embraces several different kinds of activity, but a high 94 proportion there would also indicate poor use of madvise MADV_MERGEABLE. 95 96 Izik Eidus, 97 Hugh Dickins, 17 Nov 2009