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 slightly more memory in this mode. This is the 8 default. 9 101 - Always overcommit. Appropriate for some scientific 11 applications. Classic example is code using sparse arrays 12 and just relying on the virtual memory consisting almost 13 entirely of zero pages. 14 152 - Don't overcommit. The total address space commit 16 for the system is not permitted to exceed swap + a 17 configurable percentage (default is 50) of physical RAM. 18 Depending on the percentage you use, in most situations 19 this means a process will not be killed while accessing 20 pages but will receive errors on memory allocation as 21 appropriate. 22 23 Useful for applications that want to guarantee their 24 memory allocations will be available in the future 25 without having to initialize every page. 26 27The overcommit policy is set via the sysctl `vm.overcommit_memory'. 28 29The overcommit percentage is set via `vm.overcommit_ratio'. 30 31The current overcommit limit and amount committed are viewable in 32/proc/meminfo as CommitLimit and Committed_AS respectively. 33 34Gotchas 35------- 36 37The C language stack growth does an implicit mremap. If you want absolute 38guarantees and run close to the edge you MUST mmap your stack for the 39largest size you think you will need. For typical stack usage this does 40not matter much but it's a corner case if you really really care 41 42In mode 2 the MAP_NORESERVE flag is ignored. 43 44 45How It Works 46------------ 47 48The overcommit is based on the following rules 49 50For a file backed map 51 SHARED or READ-only - 0 cost (the file is the map not swap) 52 PRIVATE WRITABLE - size of mapping per instance 53 54For an anonymous or /dev/zero map 55 SHARED - size of mapping 56 PRIVATE READ-only - 0 cost (but of little use) 57 PRIVATE WRITABLE - size of mapping per instance 58 59Additional accounting 60 Pages made writable copies by mmap 61 shmfs memory drawn from the same pool 62 63Status 64------ 65 66o We account mmap memory mappings 67o We account mprotect changes in commit 68o We account mremap changes in size 69o We account brk 70o We account munmap 71o We report the commit status in /proc 72o Account and check on fork 73o Review stack handling/building on exec 74o SHMfs accounting 75o Implement actual limit enforcement 76 77To Do 78----- 79o Account ptrace pages (this is hard) 80