1The Linux kernel supports the following overcommit handling modes 2 30 - 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. 9 101 - Always overcommit. Appropriate for some scientific 11 applications. 12 132 - 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 21The overcommit policy is set via the sysctl `vm.overcommit_memory'. 22 23The overcommit percentage is set via `vm.overcommit_ratio'. 24 25The current overcommit limit and amount committed are viewable in 26/proc/meminfo as CommitLimit and Committed_AS respectively. 27 28Gotchas 29------- 30 31The C language stack growth does an implicit mremap. If you want absolute 32guarantees and run close to the edge you MUST mmap your stack for the 33largest size you think you will need. For typical stack usage this does 34not matter much but it's a corner case if you really really care 35 36In mode 2 the MAP_NORESERVE flag is ignored. 37 38 39How It Works 40------------ 41 42The overcommit is based on the following rules 43 44For 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 48For 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 53Additional accounting 54 Pages made writable copies by mmap 55 shmfs memory drawn from the same pool 56 57Status 58------ 59 60o We account mmap memory mappings 61o We account mprotect changes in commit 62o We account mremap changes in size 63o We account brk 64o We account munmap 65o We report the commit status in /proc 66o Account and check on fork 67o Review stack handling/building on exec 68o SHMfs accounting 69o Implement actual limit enforcement 70 71To Do 72----- 73o Account ptrace pages (this is hard) 74