projects / deliverable / linux.git / blame
| Commit | Line | Data |
|---|---|---|
| 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) |