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 absoluteguarantees 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 doesnot 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------ o	We account mmap memory mappingso	We account mprotect changes in commito	We account mremap changes in sizeo	We account brko	We account munmapo	We report the commit status in /proco	Account and check on forko	Review stack handling/building on execo	SHMfs accountingo	Implement actual limit enforcement To Do-----o	Account ptrace pages (this is hard)