OSDev.org

I would like to add, that in case of huge memory allocations, there may arise a scenario where you have to over commit memory. This is where you assume that there will be plenty of processes that do not use all the memory they asked for and allocate, for all of them collectively, more than there is physically present on the machine. The problem is that you do not know who to kill when many processes write to memory, that is rightfully in their possession, and collectively use it all out.

This analogy illustrates well why I think over committing is a ridiculously bad idea. My point here is that you can allocate for something that is not yet in the memory, but you should not allocate memory/swap, that isn't there at all.