What are the benefits of Virtual Memory?

Question

So I've been reading up on virtual memory and it all seems pretty straight-forward and practical. However, I'm choking on one major concept.

If virtual memory is stored in main memory, then why even use it in the first place? Why not just use main memory?

I'm sure I'm missing something here, just don't understand what yet.

Answer 1

Virtual memory can be stored in RAM, but it can be swapped out to disk when another process needs the physical RAM. This is one of the significant features. Once it is on disk, other processes can use the system RAM to speed their processing. When needed, the memory swapped to disk can be reloaded and something else moved their in its place.

Many of today's computers have more RAM than they need so swapping is minimized (it can hurt performance), but it is good to have the swap option when needed.

Answer 2

Virtual memory allows you to concurrently run 10 x 1 GB applications in a 2 GB computer.

Demand Paging

Instead of loading the program into memory and giving it some memory for data, VM allocates space on disk (the paging file, or equivalent) for the memory needed by the program but uses "demand paging" to only load into real memory the smaller part of the program and data needed at any point. Less used parts (e.g. some rarely used subroutines) might never be loaded into real memory. When real memory is needed for another program, unused parts of real memory are used, if none, oldest clean pages may be dropped, if no clean pages remain, some "dirty pages" of memory are written out to the paging file (if the page is the same as the original loaded from the program binary file, we don't need to save an extra copy). VM keeps track of all this.

Swapping

In the past, a separate and more desperate method of memory management called swapping was also sometimes needed. Nowadays the terms are used almost synonymously.