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). These only have an effect 38when ``vm.overcommit_memory`` is set to 2. 39 40The current overcommit limit and amount committed are viewable in 41``/proc/meminfo`` as CommitLimit and Committed_AS respectively. 42 43Gotchas 44======= 45 46The C language stack growth does an implicit mremap. If you want absolute 47guarantees and run close to the edge you MUST mmap your stack for the 48largest size you think you will need. For typical stack usage this does 49not matter much but it's a corner case if you really really care 50 51In mode 2 the MAP_NORESERVE flag is ignored. 52 53 54How It Works 55============ 56 57The overcommit is based on the following rules 58 59For a file backed map 60 | SHARED or READ-only - 0 cost (the file is the map not swap) 61 | PRIVATE WRITABLE - size of mapping per instance 62 63For an anonymous or ``/dev/zero`` map 64 | SHARED - size of mapping 65 | PRIVATE READ-only - 0 cost (but of little use) 66 | PRIVATE WRITABLE - size of mapping per instance 67 68Additional accounting 69 | Pages made writable copies by mmap 70 | shmfs memory drawn from the same pool 71 72Status 73====== 74 75* We account mmap memory mappings 76* We account mprotect changes in commit 77* We account mremap changes in size 78* We account brk 79* We account munmap 80* We report the commit status in /proc 81* Account and check on fork 82* Review stack handling/building on exec 83* SHMfs accounting 84* Implement actual limit enforcement 85 86To Do 87===== 88* Account ptrace pages (this is hard) 89