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Based on kernel version 3.13. Page generated on 2014-01-20 22:05 EST.

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)
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