How can an application use more Real Memory than Virtual memory?

I currently have an application in Mac Os X whose Real Memory footprint grows while running, but it's virtual memory size (i.e. used address space) remains around the same value (20 mb).

Also total "live bytes" value in Instruments "Allocations" tool report a stable value, so I can't debug where the memory leak 开发者_运维百科is from.

In what situation is that possible?

I don't think the leak it's a reporting error in Activity Monitor since the system starts swapping and become unresponsive if the memory grows over the physical memory, and same issue is present in the Windows port of the same application (though i didn't try any memory debugger in Windows version).

looks like there was a vertex buffer leak in opengl code. Apparently the memory allocated in this way doesn't use address space. I still don't understand how it can be like this but actually it is.