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The windows XP tasks manager can show two different columns regarding the memory usage of the processes. One is called Mem Usage and the other is VM Size (not on by default, you need to activate it)

From what I've gathered, VM size is the size of the entire memory space occupied by the process and Mem Usage is the amount of memory currently committed and used. This assumption is verified by most processes when the VM Size is only slightly larger than Mem Usage for instance my Outlook currently has 79,724 K in VM Size and 56,600 K in Mem Usage
But it fails for other processes such as Firefox which currently has 171,900 K for Mem Usage and only 156,440 K in VM Size. How can a process use more memory than the amount of virtual memory allocated to it?

So Maybe my interpretation of these columns is wrong. What do they actually mean?

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5 Answers

From the Task Manager Help topics:

Memory Usage
In Task Manager, the current working set of a process, in kilobytes. The current working set is the number of pages currently resident in memory. On the Task Manager Processes tab, the column heading is Mem Usage.

Virtual Memory Size In Task Manager, the amount of virtual memory, or address space, committed to a process.

So Mem Usage is the amount of physical RAM currently allocated to a process, and VM Size is the amount of virtual (disk-based) memory currently allocated to a process.

These two counters are independent, although a process that shows a high number in one will usually show a high number for the other. It's just that there's no specific relationship between the two.

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The definition of "virtual memory" is based on redefining the address space with a contiguous virtual memory addresses to "trick" programs into thinking they are using large blocks of contiguous addresses. source

So sometimes there may be stuff from older tabs laying in the VM address space. For isntance I have 50 tabs open in my browser session right now. If I click on the very first tab chances are that it's sitting in VM waiting to be called on. So it will take a second to draw on the screen. I start to hear the hard drive churn away while it gets that tab out of VM

If that isn't the case. It may be that FF is leaking memory or a extension is the culprit. I had issue like there with the divx web player plugin on a beta version of firefox.

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Just guessing, may it be shared memory blocks that were allocated by Firefox, then given away to some other task and unmapped by FF but still counted as owned by it?

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The only explanation I see is memory fragmentation. When the process allocates 1 octet, it counts for 1 octet in the VM size. But this 1 octet occupies a page of the physical memory: for windows operating system, a page is 4K. So if the process memory is fragmented and uses a lot of small blocks it can lead to a huge physical memory usage compares to the real memory size allocated.

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VM size in task manager refers to how much it is paging to the actual virtual memory (swap file) on disk. Linky

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