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1da177e4 LT |
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 | |
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) |