Kernel Samepage Merging

Overview

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 https://lwn.net/Articles/330589/

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.

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). The amount of pages that KSM daemon scans in a single pass and the time between the passes are configured using sysfs intraface

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).

Controlling KSM with madvise

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 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 for /proc/sys/vm/).

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.

KSM daemon sysfs interface

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 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 the 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 to 0 to stop ksmd from running but keep merged pages,

• set to 1 to run ksmd e.g. echo 1 > /sys/kernel/mm/ksm/run,

• set to 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 high latency for virtual memory operations that involve traversal of the virtual mappings that share the KSM page. The minimum value is 2 as a newly created KSM page will have at least two sharers. The higher this value the faster KSM will merge the memory and the higher the deduplication factor will be, but the slower the worst case virtual mappings traversal could be for any given KSM page. Slowing down this traversal 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 virtual mappings traversal is not affected by this parameter as these traversals are always schedule friendly themselves.

stable_node_chains_prune_millisecs

specifies how frequently KSM checks the metadata of the pages that hit the deduplication limit for stale information. Smaller milllisecs values will free up the KSM metadata with lower latency, but they will make ksmd use more CPU during the scan. It’s a noop if not a single KSM page hit the max_page_sharing yet.

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

the number of KSM pages that hit the max_page_sharing limit

stable_node_dups

number of duplicated KSM pages

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 pages_sharing/pages_shared ratio is limited by the max_page_sharing tunable. To increase the ratio max_page_sharing must be increased accordingly.

Monitoring KSM profit

KSM can save memory by merging identical pages, but also can consume additional memory, because it needs to generate a number of rmap_items to save each scanned page’s brief rmap information. Some of these pages may be merged, but some may not be abled to be merged after being checked several times, which are unprofitable memory consumed.

1. How to determine whether KSM save memory or consume memory in system-wide range? Here is a simple approximate calculation for reference:

   general_profit =~ pages_sharing * sizeof(page) - (all_rmap_items) *
                     sizeof(rmap_item);
   

   where all_rmap_items can be easily obtained by summing pages_sharing, pages_shared, pages_unshared and pages_volatile.

2. The KSM profit inner a single process can be similarly obtained by the following approximate calculation:

   process_profit =~ ksm_merging_pages * sizeof(page) -
                     ksm_rmap_items * sizeof(rmap_item).
   

   where ksm_merging_pages is shown under the directory /proc/<pid>/, and ksm_rmap_items is shown in /proc/<pid>/ksm_stat.

From the perspective of application, a high ratio of ksm_rmap_items to ksm_merging_pages means a bad madvise-applied policy, so developers or administrators have to rethink how to change madvise policy. Giving an example for reference, a page’s size is usually 4K, and the rmap_item’s size is separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture. so if the ksm_rmap_items/ksm_merging_pages ratio exceeds 64 on 64-bit CPU or exceeds 128 on 32-bit CPU, then the app’s madvise policy should be dropped, because the ksm profit is approximately zero or negative.

Monitoring KSM events

There are some counters in /proc/vmstat that may be used to monitor KSM events. KSM might help save memory, it’s a tradeoff by may suffering delay on KSM COW or on swapping in copy. Those events could help users evaluate whether or how to use KSM. For example, if cow_ksm increases too fast, user may decrease the range of madvise(, , MADV_MERGEABLE).

cow_ksm

is incremented every time a KSM page triggers copy on write (COW) when users try to write to a KSM page, we have to make a copy.

ksm_swpin_copy

is incremented every time a KSM page is copied when swapping in note that KSM page might be copied when swapping in because do_swap_page() cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.

– Izik Eidus, Hugh Dickins, 17 Nov 2009