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IIS : Virtual Memory

About Virtual Memory

Virtual Memory is not the same as memory stored on disk in the page file.
Virtual Memory is a concept of indirection introduced in memory addressing whereby application see a contiguous "virtual memory" address space, but the OS transparently fulfills that memory address space with EITHER physical memory (i.e. RAM) or something else like disk memory (i.e. pagefile).
From the application's perspective, it is simply using "memory". However, the OS knows whether that "memory" is real physical memory or has to be swapped in from the pagefile on disk into real memory. It is this swapping operation which makes virtual memory that is in the page file slow.
If you want more information on the subject, I suggest reading books like "Inside Windows 2000" or "Microsoft Windows Internals" by Solomon and Russinovich, or classic books like "Computer Architectures: A Quantitative Approach". I know that it's probably way more information and detail than you are asking for, but those are the authoritative sources that I go with...

Memory Fragmentation vs Memory Leak

The classic signs of Memory Fragmentation is if you have lots of physical RAM free, low committed memory, yet seeing Out Of Memory errors.
How to distinguish this from Memory leak? Well, Memory leak would consume your physical RAM such that very little is free, virtual memory would be maxed out, and you see Out Of Memory errors. When you see the error depends on when your Virtual Memory depletes - a pagefile simply delays the inevitable and eventually grinds the system to a halt.
Your issue looks like memory fragmentation.
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