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linux
1=======================
2Kernel Samepage Merging
3=======================
4
5Overview
6========
7
8KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y,
9added to the Linux kernel in 2.6.32. See ``mm/ksm.c`` for its implementation,
10and http://lwn.net/Articles/306704/ and https://lwn.net/Articles/330589/
11
12KSM was originally developed for use with KVM (where it was known as
13Kernel Shared Memory), to fit more virtual machines into physical memory,
14by sharing the data common between them. But it can be useful to any
15application which generates many instances of the same data.
16
17The KSM daemon ksmd periodically scans those areas of user memory
18which have been registered with it, looking for pages of identical
19content which can be replaced by a single write-protected page (which
20is automatically copied if a process later wants to update its
21content). The amount of pages that KSM daemon scans in a single pass
22and the time between the passes are configured using :ref:`sysfs
23interface <ksm_sysfs>`
24
25KSM only merges anonymous (private) pages, never pagecache (file) pages.
26KSM's merged pages were originally locked into kernel memory, but can now
27be swapped out just like other user pages (but sharing is broken when they
28are swapped back in: ksmd must rediscover their identity and merge again).
29
30Controlling KSM with madvise
31============================
32
33KSM only operates on those areas of address space which an application
34has advised to be likely candidates for merging, by using the madvise(2)
35system call::
36
37 int madvise(addr, length, MADV_MERGEABLE)
38
39The app may call
40
41::
42
43 int madvise(addr, length, MADV_UNMERGEABLE)
44
45to cancel that advice and restore unshared pages: whereupon KSM
46unmerges whatever it merged in that range. Note: this unmerging call
47may suddenly require more memory than is available - possibly failing
48with EAGAIN, but more probably arousing the Out-Of-Memory killer.
49
50If KSM is not configured into the running kernel, madvise MADV_MERGEABLE
51and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was
52built with CONFIG_KSM=y, those calls will normally succeed: even if the
53KSM daemon is not currently running, MADV_MERGEABLE still registers
54the range for whenever the KSM daemon is started; even if the range
55cannot contain any pages which KSM could actually merge; even if
56MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE.
57
58If a region of memory must be split into at least one new MADV_MERGEABLE
59or MADV_UNMERGEABLE region, the madvise may return ENOMEM if the process
60will exceed ``vm.max_map_count`` (see Documentation/admin-guide/sysctl/vm.rst).
61
62Like other madvise calls, they are intended for use on mapped areas of
63the user address space: they will report ENOMEM if the specified range
64includes unmapped gaps (though working on the intervening mapped areas),
65and might fail with EAGAIN if not enough memory for internal structures.
66
67Applications should be considerate in their use of MADV_MERGEABLE,
68restricting its use to areas likely to benefit. KSM's scans may use a lot
69of processing power: some installations will disable KSM for that reason.
70
71.. _ksm_sysfs:
72
73KSM daemon sysfs interface
74==========================
75
76The KSM daemon is controlled by sysfs files in ``/sys/kernel/mm/ksm/``,
77readable by all but writable only by root:
78
79pages_to_scan
80 how many pages to scan before ksmd goes to sleep
81 e.g. ``echo 100 > /sys/kernel/mm/ksm/pages_to_scan``.
82
83 Default: 100 (chosen for demonstration purposes)
84
85sleep_millisecs
86 how many milliseconds ksmd should sleep before next scan
87 e.g. ``echo 20 > /sys/kernel/mm/ksm/sleep_millisecs``
88
89 Default: 20 (chosen for demonstration purposes)
90
91merge_across_nodes
92 specifies if pages from different NUMA nodes can be merged.
93 When set to 0, ksm merges only pages which physically reside
94 in the memory area of same NUMA node. That brings lower
95 latency to access of shared pages. Systems with more nodes, at
96 significant NUMA distances, are likely to benefit from the
97 lower latency of setting 0. Smaller systems, which need to
98 minimize memory usage, are likely to benefit from the greater
99 sharing of setting 1 (default). You may wish to compare how
100 your system performs under each setting, before deciding on
101 which to use. ``merge_across_nodes`` setting can be changed only
102 when there are no ksm shared pages in the system: set run 2 to
103 unmerge pages first, then to 1 after changing
104 ``merge_across_nodes``, to remerge according to the new setting.
105
106 Default: 1 (merging across nodes as in earlier releases)
107
108run
109 * set to 0 to stop ksmd from running but keep merged pages,
110 * set to 1 to run ksmd e.g. ``echo 1 > /sys/kernel/mm/ksm/run``,
111 * set to 2 to stop ksmd and unmerge all pages currently merged, but
112 leave mergeable areas registered for next run.
113
114 Default: 0 (must be changed to 1 to activate KSM, except if
115 CONFIG_SYSFS is disabled)
116
117use_zero_pages
118 specifies whether empty pages (i.e. allocated pages that only
119 contain zeroes) should be treated specially. When set to 1,
120 empty pages are merged with the kernel zero page(s) instead of
121 with each other as it would happen normally. This can improve
122 the performance on architectures with coloured zero pages,
123 depending on the workload. Care should be taken when enabling
124 this setting, as it can potentially degrade the performance of
125 KSM for some workloads, for example if the checksums of pages
126 candidate for merging match the checksum of an empty
127 page. This setting can be changed at any time, it is only
128 effective for pages merged after the change.
129
130 Default: 0 (normal KSM behaviour as in earlier releases)
131
132max_page_sharing
133 Maximum sharing allowed for each KSM page. This enforces a
134 deduplication limit to avoid high latency for virtual memory
135 operations that involve traversal of the virtual mappings that
136 share the KSM page. The minimum value is 2 as a newly created
137 KSM page will have at least two sharers. The higher this value
138 the faster KSM will merge the memory and the higher the
139 deduplication factor will be, but the slower the worst case
140 virtual mappings traversal could be for any given KSM
141 page. Slowing down this traversal means there will be higher
142 latency for certain virtual memory operations happening during
143 swapping, compaction, NUMA balancing and page migration, in
144 turn decreasing responsiveness for the caller of those virtual
145 memory operations. The scheduler latency of other tasks not
146 involved with the VM operations doing the virtual mappings
147 traversal is not affected by this parameter as these
148 traversals are always schedule friendly themselves.
149
150stable_node_chains_prune_millisecs
151 specifies how frequently KSM checks the metadata of the pages
152 that hit the deduplication limit for stale information.
153 Smaller milllisecs values will free up the KSM metadata with
154 lower latency, but they will make ksmd use more CPU during the
155 scan. It's a noop if not a single KSM page hit the
156 ``max_page_sharing`` yet.
157
158The effectiveness of KSM and MADV_MERGEABLE is shown in ``/sys/kernel/mm/ksm/``:
159
160general_profit
161 how effective is KSM. The calculation is explained below.
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 profit
188=====================
189
190KSM can save memory by merging identical pages, but also can consume
191additional memory, because it needs to generate a number of rmap_items to
192save each scanned page's brief rmap information. Some of these pages may
193be merged, but some may not be abled to be merged after being checked
194several times, which are unprofitable memory consumed.
195
1961) How to determine whether KSM save memory or consume memory in system-wide
197 range? Here is a simple approximate calculation for reference::
198
199 general_profit =~ pages_sharing * sizeof(page) - (all_rmap_items) *
200 sizeof(rmap_item);
201
202 where all_rmap_items can be easily obtained by summing ``pages_sharing``,
203 ``pages_shared``, ``pages_unshared`` and ``pages_volatile``.
204
2052) The KSM profit inner a single process can be similarly obtained by the
206 following approximate calculation::
207
208 process_profit =~ ksm_merging_pages * sizeof(page) -
209 ksm_rmap_items * sizeof(rmap_item).
210
211 where ksm_merging_pages is shown under the directory ``/proc/<pid>/``,
212 and ksm_rmap_items is shown in ``/proc/<pid>/ksm_stat``. The process profit
213 is also shown in ``/proc/<pid>/ksm_stat`` as ksm_process_profit.
214
215From the perspective of application, a high ratio of ``ksm_rmap_items`` to
216``ksm_merging_pages`` means a bad madvise-applied policy, so developers or
217administrators have to rethink how to change madvise policy. Giving an example
218for reference, a page's size is usually 4K, and the rmap_item's size is
219separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
220so if the ``ksm_rmap_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU
221or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
222because the ksm profit is approximately zero or negative.
223
224Monitoring KSM events
225=====================
226
227There are some counters in /proc/vmstat that may be used to monitor KSM events.
228KSM might help save memory, it's a tradeoff by may suffering delay on KSM COW
229or on swapping in copy. Those events could help users evaluate whether or how
230to use KSM. For example, if cow_ksm increases too fast, user may decrease the
231range of madvise(, , MADV_MERGEABLE).
232
233cow_ksm
234 is incremented every time a KSM page triggers copy on write (COW)
235 when users try to write to a KSM page, we have to make a copy.
236
237ksm_swpin_copy
238 is incremented every time a KSM page is copied when swapping in
239 note that KSM page might be copied when swapping in because do_swap_page()
240 cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.
241
242--
243Izik Eidus,
244Hugh Dickins, 17 Nov 2009