| 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. |
| |
| If a region of memory must be split into at least one new MADV_MERGEABLE |
| or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process |
| will exceed vm.max_map_count (see Documentation/sysctl/vm.txt). |
| |
| 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) |
| |
| use_zero_pages - specifies whether empty pages (i.e. allocated pages |
| that only contain zeroes) should be treated specially. |
| When set to 1, empty pages are merged with the kernel |
| zero page(s) instead of with each other as it would |
| happen normally. This can improve the performance on |
| architectures with coloured zero pages, depending on |
| the workload. Care should be taken when enabling this |
| setting, as it can potentially degrade the performance |
| of KSM for some workloads, for example if the checksums |
| of pages candidate for merging match the checksum of |
| an empty page. This setting can be changed at any time, |
| it is only effective for pages merged after the change. |
| Default: 0 (normal KSM behaviour as in earlier releases) |
| |
| max_page_sharing - Maximum sharing allowed for each KSM page. This |
| enforces a deduplication limit to avoid the virtual |
| memory rmap lists to grow too large. The minimum |
| value is 2 as a newly created KSM page will have at |
| least two sharers. The rmap walk has O(N) |
| complexity where N is the number of rmap_items |
| (i.e. virtual mappings) that are sharing the page, |
| which is in turn capped by max_page_sharing. So |
| this effectively spread the the linear O(N) |
| computational complexity from rmap walk context |
| over different KSM pages. The ksmd walk over the |
| stable_node "chains" is also O(N), but N is the |
| number of stable_node "dups", not the number of |
| rmap_items, so it has not a significant impact on |
| ksmd performance. In practice the best stable_node |
| "dup" candidate will be kept and found at the head |
| of the "dups" list. The higher this value the |
| faster KSM will merge the memory (because there |
| will be fewer stable_node dups queued into the |
| stable_node chain->hlist to check for pruning) and |
| the higher the deduplication factor will be, but |
| the slowest the worst case rmap walk could be for |
| any given KSM page. Slowing down the rmap_walk |
| means there will be higher latency for certain |
| virtual memory operations happening during |
| swapping, compaction, NUMA balancing and page |
| migration, in turn decreasing responsiveness for |
| the caller of those virtual memory operations. The |
| scheduler latency of other tasks not involved with |
| the VM operations doing the rmap walk is not |
| affected by this parameter as the rmap walks are |
| always schedule friendly themselves. |
| |
| stable_node_chains_prune_millisecs - How frequently to walk the whole |
| list of stable_node "dups" linked in the |
| stable_node "chains" in order to prune stale |
| stable_nodes. Smaller milllisecs values will free |
| up the KSM metadata with lower latency, but they |
| will make ksmd use more CPU during the scan. This |
| only applies to the stable_node chains so it's a |
| noop if not a single KSM page hit the |
| max_page_sharing yet (there would be no stable_node |
| chains in such case). |
| |
| 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 |
| |
| stable_node_chains - number of stable node chains allocated, this is |
| effectively the number of KSM pages that hit the |
| max_page_sharing limit |
| stable_node_dups - number of stable node dups queued into the |
| stable_node chains |
| |
| 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. |
| |
| The maximum possible page_sharing/page_shared ratio is limited by the |
| max_page_sharing tunable. To increase the ratio max_page_sharing must |
| be increased accordingly. |
| |
| The stable_node_dups/stable_node_chains ratio is also affected by the |
| max_page_sharing tunable, and an high ratio may indicate fragmentation |
| in the stable_node dups, which could be solved by introducing |
| fragmentation algorithms in ksmd which would refile rmap_items from |
| one stable_node dup to another stable_node dup, in order to freeup |
| stable_node "dups" with few rmap_items in them, but that may increase |
| the ksmd CPU usage and possibly slowdown the readonly computations on |
| the KSM pages of the applications. |
| |
| Izik Eidus, |
| Hugh Dickins, 17 Nov 2009 |
