Saving results to disk requires disk buffers, which can require the kernel to allocate, which won't work on an OOM system. This is how you get deadlocks: process trying to "handle" OOM ends up trying to allocate, and the system freezes. Which is why the OOM killer exists.

I should have said "underutilized" above. But the point is very real: applications map stuff all over the place. It's quite rare that they actually fault in those pages.

The stack thing is the obvious example: measure the stack size of your application (easy trick is to subtract the address of a local variable "deep" in the tree from the address of one of the arguments to main, or pthread_create, or wherever the top is). Now check the size of the mapping that contains the stack. And tell me if you want to actually reserve all that memory.