1.. _overcommit_accounting:
2
3=====================
4Overcommit Accounting
5=====================
6
7The Linux kernel supports the following overcommit handling modes
8
90
10	Heuristic overcommit handling. Obvious overcommits of address
11	space are refused. Used for a typical system. It ensures a
12	seriously wild allocation fails while allowing overcommit to
13	reduce swap usage.  root is allowed to allocate slightly more
14	memory in this mode. This is the default.
15
161
17	Always overcommit. Appropriate for some scientific
18	applications. Classic example is code using sparse arrays and
19	just relying on the virtual memory consisting almost entirely
20	of zero pages.
21
222
23	Don't overcommit. The total address space commit for the
24	system is not permitted to exceed swap + a configurable amount
25	(default is 50%) of physical RAM.  Depending on the amount you
26	use, in most situations this means a process will not be
27	killed while accessing pages but will receive errors on memory
28	allocation as appropriate.
29
30	Useful for applications that want to guarantee their memory
31	allocations will be available in the future without having to
32	initialize every page.
33
34The overcommit policy is set via the sysctl ``vm.overcommit_memory``.
35
36The overcommit amount can be set via ``vm.overcommit_ratio`` (percentage)
37or ``vm.overcommit_kbytes`` (absolute value).
38
39The current overcommit limit and amount committed are viewable in
40``/proc/meminfo`` as CommitLimit and Committed_AS respectively.
41
42Gotchas
43=======
44
45The C language stack growth does an implicit mremap. If you want absolute
46guarantees and run close to the edge you MUST mmap your stack for the
47largest size you think you will need. For typical stack usage this does
48not matter much but it's a corner case if you really really care
49
50In mode 2 the MAP_NORESERVE flag is ignored.
51
52
53How It Works
54============
55
56The overcommit is based on the following rules
57
58For a file backed map
59	| SHARED or READ-only	-	0 cost (the file is the map not swap)
60	| PRIVATE WRITABLE	-	size of mapping per instance
61
62For an anonymous or ``/dev/zero`` map
63	| SHARED			-	size of mapping
64	| PRIVATE READ-only	-	0 cost (but of little use)
65	| PRIVATE WRITABLE	-	size of mapping per instance
66
67Additional accounting
68	| Pages made writable copies by mmap
69	| shmfs memory drawn from the same pool
70
71Status
72======
73
74*	We account mmap memory mappings
75*	We account mprotect changes in commit
76*	We account mremap changes in size
77*	We account brk
78*	We account munmap
79*	We report the commit status in /proc
80*	Account and check on fork
81*	Review stack handling/building on exec
82*	SHMfs accounting
83*	Implement actual limit enforcement
84
85To Do
86=====
87*	Account ptrace pages (this is hard)
88