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1 | How to use the Kernel Samepage Merging feature |
2 | ---------------------------------------------- | |
3 | ||
4 | KSM is a memory-saving de-duplication feature, enabled by CONFIG_KSM=y, | |
5 | added to the Linux kernel in 2.6.32. See mm/ksm.c for its implementation, | |
6 | and http://lwn.net/Articles/306704/ and http://lwn.net/Articles/330589/ | |
7 | ||
8 | The KSM daemon ksmd periodically scans those areas of user memory which | |
9 | have been registered with it, looking for pages of identical content which | |
10 | can be replaced by a single write-protected page (which is automatically | |
11 | copied if a process later wants to update its content). | |
12 | ||
13 | KSM was originally developed for use with KVM (where it was known as | |
14 | Kernel Shared Memory), to fit more virtual machines into physical memory, | |
15 | by sharing the data common between them. But it can be useful to any | |
16 | application which generates many instances of the same data. | |
17 | ||
18 | KSM only merges anonymous (private) pages, never pagecache (file) pages. | |
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19 | KSM's merged pages were originally locked into kernel memory, but can now |
20 | be swapped out just like other user pages (but sharing is broken when they | |
21 | are swapped back in: ksmd must rediscover their identity and merge again). | |
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22 | |
23 | KSM only operates on those areas of address space which an application | |
24 | has advised to be likely candidates for merging, by using the madvise(2) | |
25 | system call: int madvise(addr, length, MADV_MERGEABLE). | |
26 | ||
27 | The app may call int madvise(addr, length, MADV_UNMERGEABLE) to cancel | |
28 | that advice and restore unshared pages: whereupon KSM unmerges whatever | |
29 | it merged in that range. Note: this unmerging call may suddenly require | |
30 | more memory than is available - possibly failing with EAGAIN, but more | |
31 | probably arousing the Out-Of-Memory killer. | |
32 | ||
33 | If KSM is not configured into the running kernel, madvise MADV_MERGEABLE | |
34 | and MADV_UNMERGEABLE simply fail with EINVAL. If the running kernel was | |
35 | built with CONFIG_KSM=y, those calls will normally succeed: even if the | |
36 | the KSM daemon is not currently running, MADV_MERGEABLE still registers | |
37 | the range for whenever the KSM daemon is started; even if the range | |
38 | cannot contain any pages which KSM could actually merge; even if | |
39 | MADV_UNMERGEABLE is applied to a range which was never MADV_MERGEABLE. | |
40 | ||
41 | Like other madvise calls, they are intended for use on mapped areas of | |
42 | the user address space: they will report ENOMEM if the specified range | |
43 | includes unmapped gaps (though working on the intervening mapped areas), | |
44 | and might fail with EAGAIN if not enough memory for internal structures. | |
45 | ||
46 | Applications should be considerate in their use of MADV_MERGEABLE, | |
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47 | restricting its use to areas likely to benefit. KSM's scans may use a lot |
48 | of processing power: some installations will disable KSM for that reason. | |
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49 | |
50 | The KSM daemon is controlled by sysfs files in /sys/kernel/mm/ksm/, | |
51 | readable by all but writable only by root: | |
52 | ||
7701c9c0 | 53 | pages_to_scan - how many present pages to scan before ksmd goes to sleep |
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54 | e.g. "echo 100 > /sys/kernel/mm/ksm/pages_to_scan" |
55 | Default: 100 (chosen for demonstration purposes) | |
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56 | |
57 | sleep_millisecs - how many milliseconds ksmd should sleep before next scan | |
58 | e.g. "echo 20 > /sys/kernel/mm/ksm/sleep_millisecs" | |
59 | Default: 20 (chosen for demonstration purposes) | |
60 | ||
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61 | merge_across_nodes - specifies if pages from different numa nodes can be merged. |
62 | When set to 0, ksm merges only pages which physically | |
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63 | reside in the memory area of same NUMA node. That brings |
64 | lower latency to access of shared pages. Systems with more | |
65 | nodes, at significant NUMA distances, are likely to benefit | |
66 | from the lower latency of setting 0. Smaller systems, which | |
67 | need to minimize memory usage, are likely to benefit from | |
68 | the greater sharing of setting 1 (default). You may wish to | |
69 | compare how your system performs under each setting, before | |
70 | deciding on which to use. merge_across_nodes setting can be | |
71 | changed only when there are no ksm shared pages in system: | |
72 | set run 2 to unmerge pages first, then to 1 after changing | |
73 | merge_across_nodes, to remerge according to the new setting. | |
74 | Default: 1 (merging across nodes as in earlier releases) | |
90bd6fd3 | 75 | |
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76 | run - set 0 to stop ksmd from running but keep merged pages, |
77 | set 1 to run ksmd e.g. "echo 1 > /sys/kernel/mm/ksm/run", | |
78 | set 2 to stop ksmd and unmerge all pages currently merged, | |
79 | but leave mergeable areas registered for next run | |
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80 | Default: 0 (must be changed to 1 to activate KSM, |
81 | except if CONFIG_SYSFS is disabled) | |
7701c9c0 | 82 | |
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83 | use_zero_pages - specifies whether empty pages (i.e. allocated pages |
84 | that only contain zeroes) should be treated specially. | |
85 | When set to 1, empty pages are merged with the kernel | |
86 | zero page(s) instead of with each other as it would | |
87 | happen normally. This can improve the performance on | |
88 | architectures with coloured zero pages, depending on | |
89 | the workload. Care should be taken when enabling this | |
90 | setting, as it can potentially degrade the performance | |
91 | of KSM for some workloads, for example if the checksums | |
92 | of pages candidate for merging match the checksum of | |
93 | an empty page. This setting can be changed at any time, | |
94 | it is only effective for pages merged after the change. | |
95 | Default: 0 (normal KSM behaviour as in earlier releases) | |
96 | ||
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97 | max_page_sharing - Maximum sharing allowed for each KSM page. This |
98 | enforces a deduplication limit to avoid the virtual | |
99 | memory rmap lists to grow too large. The minimum | |
100 | value is 2 as a newly created KSM page will have at | |
101 | least two sharers. The rmap walk has O(N) | |
102 | complexity where N is the number of rmap_items | |
103 | (i.e. virtual mappings) that are sharing the page, | |
104 | which is in turn capped by max_page_sharing. So | |
105 | this effectively spread the the linear O(N) | |
106 | computational complexity from rmap walk context | |
107 | over different KSM pages. The ksmd walk over the | |
108 | stable_node "chains" is also O(N), but N is the | |
109 | number of stable_node "dups", not the number of | |
110 | rmap_items, so it has not a significant impact on | |
111 | ksmd performance. In practice the best stable_node | |
112 | "dup" candidate will be kept and found at the head | |
113 | of the "dups" list. The higher this value the | |
114 | faster KSM will merge the memory (because there | |
115 | will be fewer stable_node dups queued into the | |
116 | stable_node chain->hlist to check for pruning) and | |
117 | the higher the deduplication factor will be, but | |
118 | the slowest the worst case rmap walk could be for | |
119 | any given KSM page. Slowing down the rmap_walk | |
120 | means there will be higher latency for certain | |
121 | virtual memory operations happening during | |
122 | swapping, compaction, NUMA balancing and page | |
123 | migration, in turn decreasing responsiveness for | |
124 | the caller of those virtual memory operations. The | |
125 | scheduler latency of other tasks not involved with | |
126 | the VM operations doing the rmap walk is not | |
127 | affected by this parameter as the rmap walks are | |
128 | always schedule friendly themselves. | |
129 | ||
130 | stable_node_chains_prune_millisecs - How frequently to walk the whole | |
131 | list of stable_node "dups" linked in the | |
132 | stable_node "chains" in order to prune stale | |
133 | stable_nodes. Smaller milllisecs values will free | |
134 | up the KSM metadata with lower latency, but they | |
135 | will make ksmd use more CPU during the scan. This | |
136 | only applies to the stable_node chains so it's a | |
137 | noop if not a single KSM page hit the | |
138 | max_page_sharing yet (there would be no stable_node | |
139 | chains in such case). | |
140 | ||
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141 | The effectiveness of KSM and MADV_MERGEABLE is shown in /sys/kernel/mm/ksm/: |
142 | ||
d0f209f6 | 143 | pages_shared - how many shared pages are being used |
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144 | pages_sharing - how many more sites are sharing them i.e. how much saved |
145 | pages_unshared - how many pages unique but repeatedly checked for merging | |
146 | pages_volatile - how many pages changing too fast to be placed in a tree | |
147 | full_scans - how many times all mergeable areas have been scanned | |
148 | ||
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149 | stable_node_chains - number of stable node chains allocated, this is |
150 | effectively the number of KSM pages that hit the | |
151 | max_page_sharing limit | |
152 | stable_node_dups - number of stable node dups queued into the | |
153 | stable_node chains | |
154 | ||
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155 | A high ratio of pages_sharing to pages_shared indicates good sharing, but |
156 | a high ratio of pages_unshared to pages_sharing indicates wasted effort. | |
157 | pages_volatile embraces several different kinds of activity, but a high | |
158 | proportion there would also indicate poor use of madvise MADV_MERGEABLE. | |
159 | ||
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160 | The maximum possible page_sharing/page_shared ratio is limited by the |
161 | max_page_sharing tunable. To increase the ratio max_page_sharing must | |
162 | be increased accordingly. | |
163 | ||
164 | The stable_node_dups/stable_node_chains ratio is also affected by the | |
165 | max_page_sharing tunable, and an high ratio may indicate fragmentation | |
166 | in the stable_node dups, which could be solved by introducing | |
167 | fragmentation algorithms in ksmd which would refile rmap_items from | |
168 | one stable_node dup to another stable_node dup, in order to freeup | |
169 | stable_node "dups" with few rmap_items in them, but that may increase | |
170 | the ksmd CPU usage and possibly slowdown the readonly computations on | |
171 | the KSM pages of the applications. | |
172 | ||
7701c9c0 | 173 | Izik Eidus, |
d0f209f6 | 174 | Hugh Dickins, 17 Nov 2009 |