linux/Documentation/vm/overcommit-accounting
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   1The Linux kernel supports the following overcommit handling modes
   2
   30       -       Heuristic overcommit handling. Obvious overcommits of
   4                address space are refused. Used for a typical system. It
   5                ensures a seriously wild allocation fails while allowing
   6                overcommit to reduce swap usage.  root is allowed to 
   7                allocate slightly more memory in this mode. This is the 
   8                default.
   9
  101       -       Always overcommit. Appropriate for some scientific
  11                applications.
  12
  132       -       Don't overcommit. The total address space commit
  14                for the system is not permitted to exceed swap + a
  15                configurable percentage (default is 50) of physical RAM.
  16                Depending on the percentage you use, in most situations
  17                this means a process will not be killed while accessing
  18                pages but will receive errors on memory allocation as
  19                appropriate.
  20
  21The overcommit policy is set via the sysctl `vm.overcommit_memory'.
  22
  23The overcommit percentage is set via `vm.overcommit_ratio'.
  24
  25The current overcommit limit and amount committed are viewable in
  26/proc/meminfo as CommitLimit and Committed_AS respectively.
  27
  28Gotchas
  29-------
  30
  31The C language stack growth does an implicit mremap. If you want absolute
  32guarantees and run close to the edge you MUST mmap your stack for the 
  33largest size you think you will need. For typical stack usage this does
  34not matter much but it's a corner case if you really really care
  35
  36In mode 2 the MAP_NORESERVE flag is ignored. 
  37
  38
  39How It Works
  40------------
  41
  42The overcommit is based on the following rules
  43
  44For a file backed map
  45        SHARED or READ-only     -       0 cost (the file is the map not swap)
  46        PRIVATE WRITABLE        -       size of mapping per instance
  47
  48For an anonymous or /dev/zero map
  49        SHARED                  -       size of mapping
  50        PRIVATE READ-only       -       0 cost (but of little use)
  51        PRIVATE WRITABLE        -       size of mapping per instance
  52
  53Additional accounting
  54        Pages made writable copies by mmap
  55        shmfs memory drawn from the same pool
  56
  57Status
  58------
  59
  60o       We account mmap memory mappings
  61o       We account mprotect changes in commit
  62o       We account mremap changes in size
  63o       We account brk
  64o       We account munmap
  65o       We report the commit status in /proc
  66o       Account and check on fork
  67o       Review stack handling/building on exec
  68o       SHMfs accounting
  69o       Implement actual limit enforcement
  70
  71To Do
  72-----
  73o       Account ptrace pages (this is hard)
  74
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