“Mem Usage” higher than “VM Size” in WinXP Task Manager

Question

In my Windows XP Task Manager, some processes display a higher value in the Mem Usage column than the VMSize. My Firefox instance, for example shows 111544 K as mem usage an

Answer 1

Memory fragmentation is probably the reason: If the process allocates 1 octet, it counts for 1 octet in the VMSize, but this 1 octet requires a physical page (4K on windows operating system). If after allocating/freeing memory, the process has a second octet that is separated by more than 4K from the first one, this second octet will always be stored on a separate physical page than the 1 one. So the VM Size count is 2 octets but the Memory Usage is 2 pages== 8K

So the fact that MemUsage is greater than VMSize shows that process does a lot of allocation and deallocation and fragments the memory. This could be because the process is started a long time ago. Or else there is place for optimization ;-)