Memory overcommit

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.

1
2
3

	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.