2024-09-09 08:52:07 +00:00
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The Linux kernel supports the following overcommit handling modes
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0 - Heuristic overcommit handling. Obvious overcommits of
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address space are refused. Used for a typical system. It
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ensures a seriously wild allocation fails while allowing
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overcommit to reduce swap usage. root is allowed to
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allocate slightly more memory in this mode. This is the
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default.
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1 - Always overcommit. Appropriate for some scientific
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2024-09-09 08:57:42 +00:00
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applications. Classic example is code using sparse arrays
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and just relying on the virtual memory consisting almost
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entirely of zero pages.
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2024-09-09 08:52:07 +00:00
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2 - Don't overcommit. The total address space commit
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for the system is not permitted to exceed swap + a
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2024-09-09 08:57:42 +00:00
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configurable amount (default is 50%) of physical RAM.
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Depending on the amount you use, in most situations
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2024-09-09 08:52:07 +00:00
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this means a process will not be killed while accessing
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pages but will receive errors on memory allocation as
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appropriate.
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2024-09-09 08:57:42 +00:00
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Useful for applications that want to guarantee their
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memory allocations will be available in the future
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without having to initialize every page.
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2024-09-09 08:52:07 +00:00
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The overcommit policy is set via the sysctl `vm.overcommit_memory'.
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2024-09-09 08:57:42 +00:00
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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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2024-09-09 08:52:07 +00:00
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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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o We account mmap memory mappings
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o We account mprotect changes in commit
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o We account mremap changes in size
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o We account brk
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o We account munmap
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o We report the commit status in /proc
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o Account and check on fork
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o Review stack handling/building on exec
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o SHMfs accounting
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o Implement actual limit enforcement
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To Do
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-----
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o Account ptrace pages (this is hard)
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