1.. _admin_guide_ksm: 2 3======================= 4Kernel Samepage Merging 5======================= 6 7Overview 8======== 9 10KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y, 11added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation, 12and http://lwn.net/Articles/306704/ and https://lwn.net/Articles/330589/ 13 14KSM was originally developed for use with KVM (where it was known as 15Kernel Shared Memory), to fit more virtual machines into physical memory, 16by sharing the data common between them. But it can be useful to any 17application which generates many instances of the same data. 18 19The KSM daemon ksmd periodically scans those areas of user memory 20which have been registered with it, looking for pages of identical 21content which can be replaced by a single write-protected page (which 22is automatically copied if a process later wants to update its 23content). The amount of pages that KSM daemon scans in a single pass 24and the time between the passes are configured using :ref:`sysfs 25intraface <ksm_sysfs>` 26 27KSM only merges anonymous (private) pages, never pagecache (file) pages. 28KSM's merged pages were originally locked into kernel memory, but can now 29be swapped out just like other user pages (but sharing is broken when they 30are swapped back in: ksmd must rediscover their identity and merge again). 31 32Controlling KSM with madvise 33============================ 34 35KSM only operates on those areas of address space which an application 36has advised to be likely candidates for merging, by using the madvise(2) 37system call:: 38 39 int madvise(addr, length, MADV_MERGEABLE) 40 41The app may call 42 43:: 44 45 int madvise(addr, length, MADV_UNMERGEABLE) 46 47to cancel that advice and restore unshared pages: whereupon KSM 48unmerges whatever it merged in that range. Note: this unmerging call 49may suddenly require more memory than is available - possibly failing 50with EAGAIN, but more probably arousing the Out-Of-Memory killer. 51 52If KSM is not configured into the running kernel, madvise MADV_MERGEABLE 53and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was 54built with CONFIG_KSM=y, those calls will normally succeed: even if the 55KSM daemon is not currently running, MADV_MERGEABLE still registers 56the range for whenever the KSM daemon is started; even if the range 57cannot contain any pages which KSM could actually merge; even if 58MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE. 59 60If a region of memory must be split into at least one new MADV_MERGEABLE 61or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process 62will exceed ``vm.max_map_count`` (see Documentation/admin-guide/sysctl/vm.rst). 63 64Like other madvise calls, they are intended for use on mapped areas of 65the user address space: they will report ENOMEM if the specified range 66includes unmapped gaps (though working on the intervening mapped areas), 67and might fail with EAGAIN if not enough memory for internal structures. 68 69Applications should be considerate in their use of MADV_MERGEABLE, 70restricting its use to areas likely to benefit. KSM's scans may use a lot 71of processing power: some installations will disable KSM for that reason. 72 73.. _ksm_sysfs: 74 75KSM daemon sysfs interface 76========================== 77 78The KSM daemon is controlled by sysfs files in ``/sys/kernel/mm/ksm/``, 79readable by all but writable only by root: 80 81pages_to_scan 82 how many pages to scan before ksmd goes to sleep 83 e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``. 84 85 Default: 100 (chosen for demonstration purposes) 86 87sleep_millisecs 88 how many milliseconds ksmd should sleep before next scan 89 e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs`` 90 91 Default: 20 (chosen for demonstration purposes) 92 93merge_across_nodes 94 specifies if pages from different NUMA nodes can be merged. 95 When set to 0, ksm merges only pages which physically reside 96 in the memory area of same NUMA node. That brings lower 97 latency to access of shared pages. Systems with more nodes, at 98 significant NUMA distances, are likely to benefit from the 99 lower latency of setting 0. Smaller systems, which need to 100 minimize memory usage, are likely to benefit from the greater 101 sharing of setting 1 (default). You may wish to compare how 102 your system performs under each setting, before deciding on 103 which to use. ``merge_across_nodes`` setting can be changed only 104 when there are no ksm shared pages in the system: set run 2 to 105 unmerge pages first, then to 1 after changing 106 ``merge_across_nodes``, to remerge according to the new setting. 107 108 Default: 1 (merging across nodes as in earlier releases) 109 110run 111 * set to 0 to stop ksmd from running but keep merged pages, 112 * set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``, 113 * set to 2 to stop ksmd and unmerge all pages currently merged, but 114 leave mergeable areas registered for next run. 115 116 Default: 0 (must be changed to 1 to activate KSM, except if 117 CONFIG_SYSFS is disabled) 118 119use_zero_pages 120 specifies whether empty pages (i.e. allocated pages that only 121 contain zeroes) should be treated specially. When set to 1, 122 empty pages are merged with the kernel zero page(s) instead of 123 with each other as it would happen normally. This can improve 124 the performance on architectures with coloured zero pages, 125 depending on the workload. Care should be taken when enabling 126 this setting, as it can potentially degrade the performance of 127 KSM for some workloads, for example if the checksums of pages 128 candidate for merging match the checksum of an empty 129 page. This setting can be changed at any time, it is only 130 effective for pages merged after the change. 131 132 Default: 0 (normal KSM behaviour as in earlier releases) 133 134max_page_sharing 135 Maximum sharing allowed for each KSM page. This enforces a 136 deduplication limit to avoid high latency for virtual memory 137 operations that involve traversal of the virtual mappings that 138 share the KSM page. The minimum value is 2 as a newly created 139 KSM page will have at least two sharers. The higher this value 140 the faster KSM will merge the memory and the higher the 141 deduplication factor will be, but the slower the worst case 142 virtual mappings traversal could be for any given KSM 143 page. Slowing down this traversal means there will be higher 144 latency for certain virtual memory operations happening during 145 swapping, compaction, NUMA balancing and page migration, in 146 turn decreasing responsiveness for the caller of those virtual 147 memory operations. The scheduler latency of other tasks not 148 involved with the VM operations doing the virtual mappings 149 traversal is not affected by this parameter as these 150 traversals are always schedule friendly themselves. 151 152stable_node_chains_prune_millisecs 153 specifies how frequently KSM checks the metadata of the pages 154 that hit the deduplication limit for stale information. 155 Smaller milllisecs values will free up the KSM metadata with 156 lower latency, but they will make ksmd use more CPU during the 157 scan. It's a noop if not a single KSM page hit the 158 ``max_page_sharing`` yet. 159 160The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``: 161 162pages_shared 163 how many shared pages are being used 164pages_sharing 165 how many more sites are sharing them i.e. how much saved 166pages_unshared 167 how many pages unique but repeatedly checked for merging 168pages_volatile 169 how many pages changing too fast to be placed in a tree 170full_scans 171 how many times all mergeable areas have been scanned 172stable_node_chains 173 the number of KSM pages that hit the ``max_page_sharing`` limit 174stable_node_dups 175 number of duplicated KSM pages 176 177A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good 178sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing`` 179indicates wasted effort. ``pages_volatile`` embraces several 180different kinds of activity, but a high proportion there would also 181indicate poor use of madvise MADV_MERGEABLE. 182 183The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the 184``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must 185be increased accordingly. 186 187Monitoring KSM events 188===================== 189 190There are some counters in /proc/vmstat that may be used to monitor KSM events. 191KSM might help save memory, it's a tradeoff by may suffering delay on KSM COW 192or on swapping in copy. Those events could help users evaluate whether or how 193to use KSM. For example, if cow_ksm increases too fast, user may decrease the 194range of madvise(, , MADV_MERGEABLE). 195 196cow_ksm 197 is incremented every time a KSM page triggers copy on write (COW) 198 when users try to write to a KSM page, we have to make a copy. 199 200ksm_swpin_copy 201 is incremented every time a KSM page is copied when swapping in 202 note that KSM page might be copied when swapping in because do_swap_page() 203 cannot do all the locking needed to reconstitute a cross-anon_vma KSM page. 204 205-- 206Izik Eidus, 207Hugh Dickins, 17 Nov 2009 208