<p>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 and 100576 K as VMSize. </p>

<p>According to the help file of Task Manager Mem Usage is the working set of the process and VMSize is the committed memory in the Virtual address space.</p>

<p>My question is, if the number of committed pages for a process is A and the number of pages in physical memory for the same process is B, shouldn't it always  be B ≤ A? Isn't the number of pages in physical memory per process a subset of the committed pages?</p>

<p>Or is this something to do with sharing of memory among processes? Please explain. (Perhaps my definition of 'Working Set' is off the mark).</p>

<p>Thanks.</p>

You might find some explaination in
The Memory Shell Game
Working Set
(A) – This is a set of virtual memory pages (that are committed) for a process and are located in physical RAM.  These pages fully belong to the process.  A working set is like a "currently/recently working on these pages" list.
Virtual Memory
– This is a memory that an operating system can address.  Regardless of the amount of physical RAM or hard drive space, this number is limited by your processor architecture.
Committed Memory
–  When an application touches a virtual memory page (reads/write/programmatically commits) the page becomes a committed page.  It is now backed by a physical memory page.  This will usually be a physical RAM page, but could eventually be a page in the page file on the hard disk, or it could be a page in a memory mapped file on the hard disk.  The memory manager handles the translations from the virtual memory page to the physical page.   A virtual page could be in located in physical RAM, while the page next to it could be on the hard drive in the page file.
BUT: PF (Page File) Usage - This is the total number of committed pages on the system.  It does not tell you how many are actually written to the page file.  It only tells you how much of the page file would be used if all committed pages had to be written out to the page file at the same time.
Hence B > A...
If we agree that B represents "mem usage" or also PF usage, the problem comes from the fact it actually represents
potential
page usages
: in Xp, this potential file space can be used as a place to assign those virtual memory pages that programs have asked for, but never brought into use...