| Linus Torvalds | 1da177e | 2005-04-16 15:20:36 -0700 | [diff] [blame] | 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) |