Based on kernel version 2.6.26. Page generated on 2008-07-16 21:13 EST.
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)