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Diffstat (limited to 'Documentation/vm/overcommit-accounting')
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diff --git a/Documentation/vm/overcommit-accounting b/Documentation/vm/overcommit-accounting new file mode 100644 index 000000000..cbfaaa674 --- /dev/null +++ b/Documentation/vm/overcommit-accounting @@ -0,0 +1,80 @@ +The Linux kernel supports the following overcommit handling modes + +0 - Heuristic overcommit handling. Obvious overcommits of + address space are refused. Used for a typical system. It + ensures a seriously wild allocation fails while allowing + overcommit to reduce swap usage. root is allowed to + allocate slightly more memory in this mode. This is the + default. + +1 - Always overcommit. Appropriate for some scientific + applications. Classic example is code using sparse arrays + and just relying on the virtual memory consisting almost + entirely of zero pages. + +2 - Don't overcommit. The total address space commit + for the system is not permitted to exceed swap + a + configurable amount (default is 50%) of physical RAM. + Depending on the amount you use, in most situations + this means a process will not be killed while accessing + pages but will receive errors on memory allocation as + appropriate. + + Useful for applications that want to guarantee their + memory allocations will be available in the future + without having to initialize every page. + +The overcommit policy is set via the sysctl `vm.overcommit_memory'. + +The overcommit amount can be set via `vm.overcommit_ratio' (percentage) +or `vm.overcommit_kbytes' (absolute value). + +The current overcommit limit and amount committed are viewable in +/proc/meminfo as CommitLimit and Committed_AS respectively. + +Gotchas +------- + +The C language stack growth does an implicit mremap. If you want absolute +guarantees and run close to the edge you MUST mmap your stack for the +largest size you think you will need. For typical stack usage this does +not matter much but it's a corner case if you really really care + +In mode 2 the MAP_NORESERVE flag is ignored. + + +How It Works +------------ + +The overcommit is based on the following rules + +For a file backed map + SHARED or READ-only - 0 cost (the file is the map not swap) + PRIVATE WRITABLE - size of mapping per instance + +For an anonymous or /dev/zero map + SHARED - size of mapping + PRIVATE READ-only - 0 cost (but of little use) + PRIVATE WRITABLE - size of mapping per instance + +Additional accounting + Pages made writable copies by mmap + shmfs memory drawn from the same pool + +Status +------ + +o We account mmap memory mappings +o We account mprotect changes in commit +o We account mremap changes in size +o We account brk +o We account munmap +o We report the commit status in /proc +o Account and check on fork +o Review stack handling/building on exec +o SHMfs accounting +o Implement actual limit enforcement + +To Do +----- +o Account ptrace pages (this is hard) |