.. _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). These only have an effect
when ``vm.overcommit_memory`` is set to 2.

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)
