[linux-2.6-block.git] / Documentation / vm / overcommit-accounting.rst

.. _overcommit_accounting:

=====================
Overcommit Accounting
=====================

The Linux kernel supports the following overcommit handling modes

0
	Heuristic overcommit handling. Obvious overcommits of address
	space are refused. Used for a typical system. It ensures a
	seriously wild allocation fails while allowing overcommit to
	reduce swap usage.  root is allowed to allocate slightly more
	memory in this mode. This is the default.

1
	Always overcommit. Appropriate for some scientific
	applications. Classic example is code using sparse arrays and
	just relying on the virtual memory consisting almost entirely
	of zero pages.

2
	Don't overcommit. The total address space commit for the
	system is not permitted to exceed swap + a configurable amount
	(default is 50%) of physical RAM.  Depending on the amount you
	use, in most situations this means a process will not be
	killed while accessing pages but will receive errors on memory
	allocation as appropriate.

	Useful for applications that want to guarantee their memory
	allocations will be available in the future without having to
	initialize every page.

The overcommit policy is set via the sysctl ``vm.overcommit_memory``.

The overcommit amount can be set via ``vm.overcommit_ratio`` (percentage)
or ``vm.overcommit_kbytes`` (absolute value).

The current overcommit limit and amount committed are viewable in
``/proc/meminfo`` as CommitLimit and Committed_AS respectively.

Gotchas
=======

The C language stack growth does an implicit mremap. If you want absolute
guarantees and run close to the edge you MUST mmap your stack for the
largest size you think you will need. For typical stack usage this does
not matter much but it's a corner case if you really really care

In mode 2 the MAP_NORESERVE flag is ignored.


How It Works
============

The overcommit is based on the following rules

For a file backed map
	| SHARED or READ-only	-	0 cost (the file is the map not swap)
	| PRIVATE WRITABLE	-	size of mapping per instance

For an anonymous or ``/dev/zero`` map
	| SHARED			-	size of mapping
	| PRIVATE READ-only	-	0 cost (but of little use)
	| PRIVATE WRITABLE	-	size of mapping per instance

Additional accounting
	| Pages made writable copies by mmap
	| shmfs memory drawn from the same pool

Status
======

*	We account mmap memory mappings
*	We account mprotect changes in commit
*	We account mremap changes in size
*	We account brk
*	We account munmap
*	We report the commit status in /proc
*	Account and check on fork
*	Review stack handling/building on exec
*	SHMfs accounting
*	Implement actual limit enforcement

To Do
=====
*	Account ptrace pages (this is hard)
Commit	Line	Data
	1	.. _overcommit_accounting:
	2
	3	=====================
	4	Overcommit Accounting
	5	=====================
	6
	7	The Linux kernel supports the following overcommit handling modes
	8
	9	0
	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
	16	1
	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
	22	2
	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
	34	The overcommit policy is set via the sysctl ``vm.overcommit_memory``.
	35
	36	The overcommit amount can be set via ``vm.overcommit_ratio`` (percentage)
	37	or ``vm.overcommit_kbytes`` (absolute value).
	38
	39	The current overcommit limit and amount committed are viewable in
	40	``/proc/meminfo`` as CommitLimit and Committed_AS respectively.
	41
	42	Gotchas
	43	=======
	44
	45	The C language stack growth does an implicit mremap. If you want absolute
	46	guarantees and run close to the edge you MUST mmap your stack for the
	47	largest size you think you will need. For typical stack usage this does
	48	not matter much but it's a corner case if you really really care
	49
	50	In mode 2 the MAP_NORESERVE flag is ignored.
	51
	52
	53	How It Works
	54	============
	55
	56	The overcommit is based on the following rules
	57
	58	For 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
	62	For 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
	67	Additional accounting
	68	\| Pages made writable copies by mmap
	69	\| shmfs memory drawn from the same pool
	70
	71	Status
	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
	85	To Do
	86	=====
	87	* Account ptrace pages (this is hard)