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1 | The Linux kernel supports the following overcommit handling modes |
2 | ||
3 | 0 - Heuristic overcommit handling. Obvious overcommits of | |
4 | address space are refused. Used for a typical system. It | |
5 | ensures a seriously wild allocation fails while allowing | |
6 | overcommit to reduce swap usage. root is allowed to | |
25985edc | 7 | allocate slightly more memory in this mode. This is the |
1da177e4 LT |
8 | default. |
9 | ||
10 | 1 - Always overcommit. Appropriate for some scientific | |
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11 | applications. Classic example is code using sparse arrays |
12 | and just relying on the virtual memory consisting almost | |
13 | entirely of zero pages. | |
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14 | |
15 | 2 - Don't overcommit. The total address space commit | |
16 | for the system is not permitted to exceed swap + a | |
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17 | configurable amount (default is 50%) of physical RAM. |
18 | Depending on the amount you use, in most situations | |
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19 | this means a process will not be killed while accessing |
20 | pages but will receive errors on memory allocation as | |
21 | appropriate. | |
22 | ||
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23 | Useful for applications that want to guarantee their |
24 | memory allocations will be available in the future | |
25 | without having to initialize every page. | |
26 | ||
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27 | The overcommit policy is set via the sysctl `vm.overcommit_memory'. |
28 | ||
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29 | The overcommit amount can be set via `vm.overcommit_ratio' (percentage) |
30 | or `vm.overcommit_kbytes' (absolute value). | |
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31 | |
32 | The current overcommit limit and amount committed are viewable in | |
33 | /proc/meminfo as CommitLimit and Committed_AS respectively. | |
34 | ||
35 | Gotchas | |
36 | ------- | |
37 | ||
38 | The C language stack growth does an implicit mremap. If you want absolute | |
39 | guarantees and run close to the edge you MUST mmap your stack for the | |
40 | largest size you think you will need. For typical stack usage this does | |
41 | not matter much but it's a corner case if you really really care | |
42 | ||
43 | In mode 2 the MAP_NORESERVE flag is ignored. | |
44 | ||
45 | ||
46 | How It Works | |
47 | ------------ | |
48 | ||
49 | The overcommit is based on the following rules | |
50 | ||
51 | For a file backed map | |
52 | SHARED or READ-only - 0 cost (the file is the map not swap) | |
53 | PRIVATE WRITABLE - size of mapping per instance | |
54 | ||
55 | For an anonymous or /dev/zero map | |
56 | SHARED - size of mapping | |
57 | PRIVATE READ-only - 0 cost (but of little use) | |
58 | PRIVATE WRITABLE - size of mapping per instance | |
59 | ||
60 | Additional accounting | |
61 | Pages made writable copies by mmap | |
62 | shmfs memory drawn from the same pool | |
63 | ||
64 | Status | |
65 | ------ | |
66 | ||
67 | o We account mmap memory mappings | |
68 | o We account mprotect changes in commit | |
69 | o We account mremap changes in size | |
70 | o We account brk | |
71 | o We account munmap | |
72 | o We report the commit status in /proc | |
73 | o Account and check on fork | |
74 | o Review stack handling/building on exec | |
75 | o SHMfs accounting | |
76 | o Implement actual limit enforcement | |
77 | ||
78 | To Do | |
79 | ----- | |
80 | o Account ptrace pages (this is hard) |