Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 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 slightly more memory in this mode. This is the 8 default. 9 10 1 - 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 15 2 - Don't overcommit. The total address space commit 16 for the system is not permitted to exceed swap + a 17 configurable amount (default is 50%) of physical RAM. 18 Depending on the amount 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 27 The overcommit policy is set via the sysctl `vm.overcommit_memory'. 28 29 The overcommit amount can be set via `vm.overcommit_ratio' (percentage) 30 or `vm.overcommit_kbytes' (absolute value). 31 32 The current overcommit limit and amount committed are viewable in 33 /proc/meminfo as CommitLimit and Committed_AS respectively. 34 35 Gotchas 36 ------- 37 38 The C language stack growth does an implicit mremap. If you want absolute 39 guarantees and run close to the edge you MUST mmap your stack for the 40 largest size you think you will need. For typical stack usage this does 41 not matter much but it's a corner case if you really really care 42 43 In mode 2 the MAP_NORESERVE flag is ignored. 44 45 46 How It Works 47 ------------ 48 49 The overcommit is based on the following rules 50 51 For a file backed map 52 SHARED or READ-only - 0 cost (the file is the map not swap) 53 PRIVATE WRITABLE - size of mapping per instance 54 55 For an anonymous or /dev/zero map 56 SHARED - size of mapping 57 PRIVATE READ-only - 0 cost (but of little use) 58 PRIVATE WRITABLE - size of mapping per instance 59 60 Additional accounting 61 Pages made writable copies by mmap 62 shmfs memory drawn from the same pool 63 64 Status 65 ------ 66 67 o We account mmap memory mappings 68 o We account mprotect changes in commit 69 o We account mremap changes in size 70 o We account brk 71 o We account munmap 72 o We report the commit status in /proc 73 o Account and check on fork 74 o Review stack handling/building on exec 75 o SHMfs accounting 76 o Implement actual limit enforcement 77 78 To Do 79 ----- 80 o Account ptrace pages (this is hard)