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. Classic example is code using sparse arrays
  12                and just relying on the virtual memory consisting almost
  13                entirely of zero pages.
  14
  152       -       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
  27The overcommit policy is set via the sysctl `vm.overcommit_memory'.
  28
  29The overcommit percentage is set via `vm.overcommit_ratio'.
  30
  31The current overcommit limit and amount committed are viewable in
  32/proc/meminfo as CommitLimit and Committed_AS respectively.
  33
  34Gotchas
  35-------
  36
  37The C language stack growth does an implicit mremap. If you want absolute
  38guarantees and run close to the edge you MUST mmap your stack for the 
  39largest size you think you will need. For typical stack usage this does
  40not matter much but it's a corner case if you really really care
  41
  42In mode 2 the MAP_NORESERVE flag is ignored. 
  43
  44
  45How It Works
  46------------
  47
  48The overcommit is based on the following rules
  49
  50For 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
  54For 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
  59Additional accounting
  60        Pages made writable copies by mmap
  61        shmfs memory drawn from the same pool
  62
  63Status
  64------
  65
  66o       We account mmap memory mappings
  67o       We account mprotect changes in commit
  68o       We account mremap changes in size
  69o       We account brk
  70o       We account munmap
  71o       We report the commit status in /proc
  72o       Account and check on fork
  73o       Review stack handling/building on exec
  74o       SHMfs accounting
  75o       Implement actual limit enforcement
  76
  77To Do
  78-----
  79o       Account ptrace pages (this is hard)
  80
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