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Without a max deduplication limit for each KSM page, the list of the rmap_items associated to each stable_node can grow infinitely large. During the rmap walk each entry can take up to ~10usec to process because of IPIs for the TLB flushing (both for the primary MMU and the secondary MMUs with the MMU notifier). With only 16GB of address space shared in the same KSM page, that would amount to dozens of seconds of kernel runtime. A ~256 max deduplication factor will reduce the latencies of the rmap walks on KSM pages to order of a few msec. Just doing the cond_resched() during the rmap walks is not enough, the list size must have a limit too, otherwise the caller could get blocked in (schedule friendly) kernel computations for seconds, unexpectedly. There's room for optimization to significantly reduce the IPI delivery cost during the page_referenced(), but at least for page_migration in the KSM case (used by hard NUMA bindings, compaction and NUMA balancing) it may be inevitable to send lots of IPIs if each rmap_item->mm is active on a different CPU and there are lots of CPUs. Even if we ignore the IPI delivery cost, we've still to walk the whole KSM rmap list, so we can't allow millions or billions (ulimited) number of entries in the KSM stable_node rmap_item lists. The limit is enforced efficiently by adding a second dimension to the stable rbtree. So there are three types of stable_nodes: the regular ones (identical as before, living in the first flat dimension of the stable rbtree), the "chains" and the "dups". Every "chain" and all "dups" linked into a "chain" enforce the invariant that they represent the same write protected memory content, even if each "dup" will be pointed by a different KSM page copy of that content. This way the stable rbtree lookup computational complexity is unaffected if compared to an unlimited max_sharing_limit. It is still enforced that there cannot be KSM page content duplicates in the stable rbtree itself. Adding the second dimension to the stable rbtree only after the max_page_sharing limit hits, provides for a zero memory footprint increase on 64bit archs. The memory overhead of the per-KSM page stable_tree and per virtual mapping rmap_item is unchanged. Only after the max_page_sharing limit hits, we need to allocate a stable_tree "chain" and rb_replace() the "regular" stable_node with the newly allocated stable_node "chain". After that we simply add the "regular" stable_node to the chain as a stable_node "dup" by linking hlist_dup in the stable_node_chain->hlist. This way the "regular" (flat) stable_node is converted to a stable_node "dup" living in the second dimension of the stable rbtree. During stable rbtree lookups the stable_node "chain" is identified as stable_node->rmap_hlist_len == STABLE_NODE_CHAIN (aka is_stable_node_chain()). When dropping stable_nodes, the stable_node "dup" is identified as stable_node->head == STABLE_NODE_DUP_HEAD (aka is_stable_node_dup()). The STABLE_NODE_DUP_HEAD must be an unique valid pointer never used elsewhere in any stable_node->head/node to avoid a clashes with the stable_node->node.rb_parent_color pointer, and different from &migrate_nodes. So the second field of &migrate_nodes is picked and verified as always safe with a BUILD_BUG_ON in case the list_head implementation changes in the future. The STABLE_NODE_DUP is picked as a random negative value in stable_node->rmap_hlist_len. rmap_hlist_len cannot become negative when it's a "regular" stable_node or a stable_node "dup". The stable_node_chain->nid is irrelevant. The stable_node_chain->kpfn is aliased in a union with a time field used to rate limit the stable_node_chain->hlist prunes. The garbage collection of the stable_node_chain happens lazily during stable rbtree lookups (as for all other kind of stable_nodes), or while disabling KSM with "echo 2 >/sys/kernel/mm/ksm/run" while collecting the entire stable rbtree. While the "regular" stable_nodes and the stable_node "dups" must wait for their underlying tree_page to be freed before they can be freed themselves, the stable_node "chains" can be freed immediately if the stable_node->hlist turns empty. This is because the "chains" are never pointed by any page->mapping and they're effectively stable rbtree KSM self contained metadata. [akpm@linux-foundation.org: fix non-NUMA build] Signed-off-by: Andrea Arcangeli <aarcange@redhat.com> Tested-by: Petr Holasek <pholasek@redhat.com> Cc: Hugh Dickins <hughd@google.com> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Arjan van de Ven <arjan@linux.intel.com> Cc: Evgheni Dereveanchin <ederevea@redhat.com> Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: Gavin Guo <gavin.guo@canonical.com> Cc: Jay Vosburgh <jay.vosburgh@canonical.com> Cc: Mel Gorman <mgorman@techsingularity.net> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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178 lines
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How to use the Kernel Samepage Merging feature
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KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
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added to the Linux kernel in 2.6.32. See mm/ksm.c for its implementation,
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and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/
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The KSM daemon ksmd periodically scans those areas of user memory which
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have been registered with it, looking for pages of identical content which
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can be replaced by a single write-protected page (which is automatically
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copied if a process later wants to update its content).
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KSM was originally developed for use with KVM (where it was known as
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Kernel Shared Memory), to fit more virtual machines into physical memory,
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by sharing the data common between them. But it can be useful to any
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application which generates many instances of the same data.
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KSM only merges anonymous (private) pages, never pagecache (file) pages.
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KSM's merged pages were originally locked into kernel memory, but can now
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be swapped out just like other user pages (but sharing is broken when they
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are swapped back in: ksmd must rediscover their identity and merge again).
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KSM only operates on those areas of address space which an application
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has advised to be likely candidates for merging, by using the madvise(2)
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system call: int madvise(addr, length, MADV_MERGEABLE).
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The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel
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that advice and restore unshared pages: whereupon KSM unmerges whatever
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it merged in that range. Note: this unmerging call may suddenly require
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more memory than is available - possibly failing with EAGAIN, but more
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probably arousing the Out-Of-Memory killer.
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If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
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and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
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built with CONFIG_KSM=y, those calls will normally succeed: even if the
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the KSM daemon is not currently running, MADV_MERGEABLE still registers
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the range for whenever the KSM daemon is started; even if the range
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cannot contain any pages which KSM could actually merge; even if
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MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
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If a region of memory must be split into at least one new MADV_MERGEABLE
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or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
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will exceed vm.max_map_count (see Documentation/sysctl/vm.txt).
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Like other madvise calls, they are intended for use on mapped areas of
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the user address space: they will report ENOMEM if the specified range
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includes unmapped gaps (though working on the intervening mapped areas),
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and might fail with EAGAIN if not enough memory for internal structures.
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Applications should be considerate in their use of MADV_MERGEABLE,
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restricting its use to areas likely to benefit. KSM's scans may use a lot
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of processing power: some installations will disable KSM for that reason.
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The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/,
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readable by all but writable only by root:
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pages_to_scan - how many present pages to scan before ksmd goes to sleep
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e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan"
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Default: 100 (chosen for demonstration purposes)
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sleep_millisecs - how many milliseconds ksmd should sleep before next scan
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e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs"
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Default: 20 (chosen for demonstration purposes)
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merge_across_nodes - specifies if pages from different numa nodes can be merged.
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When set to 0, ksm merges only pages which physically
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reside in the memory area of same NUMA node. That brings
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lower latency to access of shared pages. Systems with more
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nodes, at significant NUMA distances, are likely to benefit
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from the lower latency of setting 0. Smaller systems, which
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need to minimize memory usage, are likely to benefit from
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the greater sharing of setting 1 (default). You may wish to
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compare how your system performs under each setting, before
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deciding on which to use. merge_across_nodes setting can be
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changed only when there are no ksm shared pages in system:
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set run 2 to unmerge pages first, then to 1 after changing
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merge_across_nodes, to remerge according to the new setting.
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Default: 1 (merging across nodes as in earlier releases)
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run - set 0 to stop ksmd from running but keep merged pages,
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set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run",
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set 2 to stop ksmd and unmerge all pages currently merged,
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but leave mergeable areas registered for next run
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Default: 0 (must be changed to 1 to activate KSM,
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except if CONFIG_SYSFS is disabled)
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use_zero_pages - specifies whether empty pages (i.e. allocated pages
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that only contain zeroes) should be treated specially.
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When set to 1, empty pages are merged with the kernel
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zero page(s) instead of with each other as it would
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happen normally. This can improve the performance on
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architectures with coloured zero pages, depending on
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the workload. Care should be taken when enabling this
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setting, as it can potentially degrade the performance
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of KSM for some workloads, for example if the checksums
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of pages candidate for merging match the checksum of
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an empty page. This setting can be changed at any time,
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it is only effective for pages merged after the change.
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Default: 0 (normal KSM behaviour as in earlier releases)
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max_page_sharing - Maximum sharing allowed for each KSM page. This
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enforces a deduplication limit to avoid the virtual
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memory rmap lists to grow too large. The minimum
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value is 2 as a newly created KSM page will have at
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least two sharers. The rmap walk has O(N)
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complexity where N is the number of rmap_items
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(i.e. virtual mappings) that are sharing the page,
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which is in turn capped by max_page_sharing. So
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this effectively spread the the linear O(N)
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computational complexity from rmap walk context
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over different KSM pages. The ksmd walk over the
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stable_node "chains" is also O(N), but N is the
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number of stable_node "dups", not the number of
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rmap_items, so it has not a significant impact on
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ksmd performance. In practice the best stable_node
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"dup" candidate will be kept and found at the head
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of the "dups" list. The higher this value the
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faster KSM will merge the memory (because there
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will be fewer stable_node dups queued into the
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stable_node chain->hlist to check for pruning) and
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the higher the deduplication factor will be, but
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the slowest the worst case rmap walk could be for
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any given KSM page. Slowing down the rmap_walk
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means there will be higher latency for certain
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virtual memory operations happening during
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swapping, compaction, NUMA balancing and page
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migration, in turn decreasing responsiveness for
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the caller of those virtual memory operations. The
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scheduler latency of other tasks not involved with
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the VM operations doing the rmap walk is not
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affected by this parameter as the rmap walks are
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always schedule friendly themselves.
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stable_node_chains_prune_millisecs - How frequently to walk the whole
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list of stable_node "dups" linked in the
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stable_node "chains" in order to prune stale
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stable_nodes. Smaller milllisecs values will free
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up the KSM metadata with lower latency, but they
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will make ksmd use more CPU during the scan. This
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only applies to the stable_node chains so it's a
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noop if not a single KSM page hit the
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max_page_sharing yet (there would be no stable_node
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chains in such case).
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The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/:
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pages_shared - how many shared pages are being used
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pages_sharing - how many more sites are sharing them i.e. how much saved
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pages_unshared - how many pages unique but repeatedly checked for merging
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pages_volatile - how many pages changing too fast to be placed in a tree
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full_scans - how many times all mergeable areas have been scanned
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stable_node_chains - number of stable node chains allocated, this is
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effectively the number of KSM pages that hit the
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max_page_sharing limit
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stable_node_dups - number of stable node dups queued into the
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stable_node chains
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A high ratio of pages_sharing to pages_shared indicates good sharing, but
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a high ratio of pages_unshared to pages_sharing indicates wasted effort.
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pages_volatile embraces several different kinds of activity, but a high
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proportion there would also indicate poor use of madvise MADV_MERGEABLE.
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The maximum possible page_sharing/page_shared ratio is limited by the
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max_page_sharing tunable. To increase the ratio max_page_sharing must
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be increased accordingly.
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The stable_node_dups/stable_node_chains ratio is also affected by the
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max_page_sharing tunable, and an high ratio may indicate fragmentation
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in the stable_node dups, which could be solved by introducing
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fragmentation algorithms in ksmd which would refile rmap_items from
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one stable_node dup to another stable_node dup, in order to freeup
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stable_node "dups" with few rmap_items in them, but that may increase
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the ksmd CPU usage and possibly slowdown the readonly computations on
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the KSM pages of the applications.
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Izik Eidus,
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Hugh Dickins, 17 Nov 2009
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