1.. _overcommit_accounting: 2 3===================== 4Overcommit Accounting 5===================== 6 7The Linux kernel supports the following overcommit handling modes 8 90 10 Heuristic overcommit handling. Obvious overcommits of address 11 space are refused. Used for a typical system. It ensures a 12 seriously wild allocation fails while allowing overcommit to 13 reduce swap usage. root is allowed to allocate slightly more 14 memory in this mode. This is the default. 15 161 17 Always overcommit. Appropriate for some scientific 18 applications. Classic example is code using sparse arrays and 19 just relying on the virtual memory consisting almost entirely 20 of zero pages. 21 222 23 Don't overcommit. The total address space commit for the 24 system is not permitted to exceed swap + a configurable amount 25 (default is 50%) of physical RAM. Depending on the amount you 26 use, in most situations this means a process will not be 27 killed while accessing pages but will receive errors on memory 28 allocation as appropriate. 29 30 Useful for applications that want to guarantee their memory 31 allocations will be available in the future without having to 32 initialize every page. 33 34The overcommit policy is set via the sysctl ``vm.overcommit_memory``. 35 36The overcommit amount can be set via ``vm.overcommit_ratio`` (percentage) 37or ``vm.overcommit_kbytes`` (absolute value). 38 39The current overcommit limit and amount committed are viewable in 40``/proc/meminfo`` as CommitLimit and Committed_AS respectively. 41 42Gotchas 43======= 44 45The C language stack growth does an implicit mremap. If you want absolute 46guarantees and run close to the edge you MUST mmap your stack for the 47largest size you think you will need. For typical stack usage this does 48not matter much but it's a corner case if you really really care 49 50In mode 2 the MAP_NORESERVE flag is ignored. 51 52 53How It Works 54============ 55 56The overcommit is based on the following rules 57 58For a file backed map 59 | SHARED or READ-only - 0 cost (the file is the map not swap) 60 | PRIVATE WRITABLE - size of mapping per instance 61 62For an anonymous or ``/dev/zero`` map 63 | SHARED - size of mapping 64 | PRIVATE READ-only - 0 cost (but of little use) 65 | PRIVATE WRITABLE - size of mapping per instance 66 67Additional accounting 68 | Pages made writable copies by mmap 69 | shmfs memory drawn from the same pool 70 71Status 72====== 73 74* We account mmap memory mappings 75* We account mprotect changes in commit 76* We account mremap changes in size 77* We account brk 78* We account munmap 79* We report the commit status in /proc 80* Account and check on fork 81* Review stack handling/building on exec 82* SHMfs accounting 83* Implement actual limit enforcement 84 85To Do 86===== 87* Account ptrace pages (this is hard) 88