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