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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 |
7 | allocate slighly more memory in this mode. This is the |
8 | default. |
9 | |
10 | 1 - Always overcommit. Appropriate for some scientific |
11 | applications. |
12 | |
13 | 2 - Don't overcommit. The total address space commit |
14 | for the system is not permitted to exceed swap + a |
15 | configurable percentage (default is 50) of physical RAM. |
16 | Depending on the percentage you use, in most situations |
17 | this means a process will not be killed while accessing |
18 | pages but will receive errors on memory allocation as |
19 | appropriate. |
20 | |
21 | The overcommit policy is set via the sysctl `vm.overcommit_memory'. |
22 | |
23 | The overcommit percentage is set via `vm.overcommit_ratio'. |
24 | |
25 | The current overcommit limit and amount committed are viewable in |
26 | /proc/meminfo as CommitLimit and Committed_AS respectively. |
27 | |
28 | Gotchas |
29 | ------- |
30 | |
31 | The C language stack growth does an implicit mremap. If you want absolute |
32 | guarantees and run close to the edge you MUST mmap your stack for the |
33 | largest size you think you will need. For typical stack usage this does |
34 | not matter much but it's a corner case if you really really care |
35 | |
36 | In mode 2 the MAP_NORESERVE flag is ignored. |
37 | |
38 | |
39 | How It Works |
40 | ------------ |
41 | |
42 | The overcommit is based on the following rules |
43 | |
44 | For a file backed map |
45 | SHARED or READ-only - 0 cost (the file is the map not swap) |
46 | PRIVATE WRITABLE - size of mapping per instance |
47 | |
48 | For an anonymous or /dev/zero map |
49 | SHARED - size of mapping |
50 | PRIVATE READ-only - 0 cost (but of little use) |
51 | PRIVATE WRITABLE - size of mapping per instance |
52 | |
53 | Additional accounting |
54 | Pages made writable copies by mmap |
55 | shmfs memory drawn from the same pool |
56 | |
57 | Status |
58 | ------ |
59 | |
60 | o We account mmap memory mappings |
61 | o We account mprotect changes in commit |
62 | o We account mremap changes in size |
63 | o We account brk |
64 | o We account munmap |
65 | o We report the commit status in /proc |
66 | o Account and check on fork |
67 | o Review stack handling/building on exec |
68 | o SHMfs accounting |
69 | o Implement actual limit enforcement |
70 | |
71 | To Do |
72 | ----- |
73 | o Account ptrace pages (this is hard) |
74 |
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od-2011-09-04
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v2.6.34-rc5
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