Memory overcommit

Memory overcommit
Photo by Alex Knight / Unsplash

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

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

	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.