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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.

	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.

This post is licensed under CC BY 4.0 by the author.

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