(Disclaimer: my assertions apply only to Linux, I'm not familiar with the vmm in other systems).
I think you may be misunderstanding how the vmm works. When you allocate memory by calling mmap(), sbrk() etc., that amount of memory is committed whether or not you actually use it. Because many processes will allocate more than what they actually use, for example excess stack space, the vmm allows you to overcommit by a certain percentage, defaulting to 50%. This will allow processes to allocate 50% more memory than the system actually has. If that is exceeded, however, Linux has an Out Of Memory Killer which will start killing processes.
