Documentation/vm/overcommit-accounting

   1 The Linux kernel supports the following overcommit handling modes
   2
   3 0       -       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
  10 1       -       Always overcommit. Appropriate for some scientific
  11                 applications. Classic example is code using sparse arrays
  12                 and just relying on the virtual memory consisting almost
  13                 entirely of zero pages.
  14
  15 2       -       Don't overcommit. The total address space commit
  16                 for the system is not permitted to exceed swap + a
  17                 configurable percentage (default is 50) of physical RAM.
  18                 Depending on the percentage you use, in most situations
  19                 this means a process will not be killed while accessing
  20                 pages but will receive errors on memory allocation as
  21                 appropriate.
  22
  23                 Useful for applications that want to guarantee their
  24                 memory allocations will be available in the future
  25                 without having to initialize every page.
  26
  27 The overcommit policy is set via the sysctl `vm.overcommit_memory'.
  28
  29 The overcommit percentage is set via `vm.overcommit_ratio'.
  30
  31 The current overcommit limit and amount committed are viewable in
  32 /proc/meminfo as CommitLimit and Committed_AS respectively.
  33
  34 Gotchas
  35 -------
  36
  37 The C language stack growth does an implicit mremap. If you want absolute
  38 guarantees and run close to the edge you MUST mmap your stack for the
  39 largest size you think you will need. For typical stack usage this does
  40 not matter much but it's a corner case if you really really care
  41
  42 In mode 2 the MAP_NORESERVE flag is ignored.
  43
  44
  45 How It Works
  46 ------------
  47
  48 The overcommit is based on the following rules
  49
  50 For a file backed map
  51         SHARED or READ-only     -       0 cost (the file is the map not swap)
  52         PRIVATE WRITABLE        -       size of mapping per instance
  53
  54 For an anonymous or /dev/zero map
  55         SHARED                  -       size of mapping
  56         PRIVATE READ-only       -       0 cost (but of little use)
  57         PRIVATE WRITABLE        -       size of mapping per instance
  58
  59 Additional accounting
  60         Pages made writable copies by mmap
  61         shmfs memory drawn from the same pool
  62
  63 Status
  64 ------
  65
  66 o       We account mmap memory mappings
  67 o       We account mprotect changes in commit
  68 o       We account mremap changes in size
  69 o       We account brk
  70 o       We account munmap
  71 o       We report the commit status in /proc
  72 o       Account and check on fork
  73 o       Review stack handling/building on exec
  74 o       SHMfs accounting
  75 o       Implement actual limit enforcement
  76
  77 To Do
  78 -----
  79 o       Account ptrace pages (this is hard)