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
23intraface <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
160pages_shared
161        how many shared pages are being used
162pages_sharing
163        how many more sites are sharing them i.e. how much saved
164pages_unshared
165        how many pages unique but repeatedly checked for merging
166pages_volatile
167        how many pages changing too fast to be placed in a tree
168full_scans
169        how many times all mergeable areas have been scanned
170stable_node_chains
171        the number of KSM pages that hit the ``max_page_sharing`` limit
172stable_node_dups
173        number of duplicated KSM pages
174
175A high ratio of ``pages_sharing`` to ``pages_shared`` indicates good
176sharing, but a high ratio of ``pages_unshared`` to ``pages_sharing``
177indicates wasted effort.  ``pages_volatile`` embraces several
178different kinds of activity, but a high proportion there would also
179indicate poor use of madvise MADV_MERGEABLE.
180
181The maximum possible ``pages_sharing/pages_shared`` ratio is limited by the
182``max_page_sharing`` tunable. To increase the ratio ``max_page_sharing`` must
183be increased accordingly.
184
185Monitoring KSM profit
186=====================
187
188KSM can save memory by merging identical pages, but also can consume
189additional memory, because it needs to generate a number of rmap_items to
190save each scanned page's brief rmap information. Some of these pages may
191be merged, but some may not be abled to be merged after being checked
192several times, which are unprofitable memory consumed.
193
1941) How to determine whether KSM save memory or consume memory in system-wide
195   range? Here is a simple approximate calculation for reference::
196
197	general_profit =~ pages_sharing * sizeof(page) - (all_rmap_items) *
198			  sizeof(rmap_item);
199
200   where all_rmap_items can be easily obtained by summing ``pages_sharing``,
201   ``pages_shared``, ``pages_unshared`` and ``pages_volatile``.
202
2032) The KSM profit inner a single process can be similarly obtained by the
204   following approximate calculation::
205
206	process_profit =~ ksm_merging_pages * sizeof(page) -
207			  ksm_rmap_items * sizeof(rmap_item).
208
209   where ksm_merging_pages is shown under the directory ``/proc/<pid>/``,
210   and ksm_rmap_items is shown in ``/proc/<pid>/ksm_stat``.
211
212From the perspective of application, a high ratio of ``ksm_rmap_items`` to
213``ksm_merging_pages`` means a bad madvise-applied policy, so developers or
214administrators have to rethink how to change madvise policy. Giving an example
215for reference, a page's size is usually 4K, and the rmap_item's size is
216separately 32B on 32-bit CPU architecture and 64B on 64-bit CPU architecture.
217so if the ``ksm_rmap_items/ksm_merging_pages`` ratio exceeds 64 on 64-bit CPU
218or exceeds 128 on 32-bit CPU, then the app's madvise policy should be dropped,
219because the ksm profit is approximately zero or negative.
220
221Monitoring KSM events
222=====================
223
224There are some counters in /proc/vmstat that may be used to monitor KSM events.
225KSM might help save memory, it's a tradeoff by may suffering delay on KSM COW
226or on swapping in copy. Those events could help users evaluate whether or how
227to use KSM. For example, if cow_ksm increases too fast, user may decrease the
228range of madvise(, , MADV_MERGEABLE).
229
230cow_ksm
231	is incremented every time a KSM page triggers copy on write (COW)
232	when users try to write to a KSM page, we have to make a copy.
233
234ksm_swpin_copy
235	is incremented every time a KSM page is copied when swapping in
236	note that KSM page might be copied when swapping in because do_swap_page()
237	cannot do all the locking needed to reconstitute a cross-anon_vma KSM page.
238
239--
240Izik Eidus,
241Hugh Dickins, 17 Nov 2009
242