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What do the columns that are not shown by default in the XP Task Manager do? Specifically the VM Size, Page Faults and Mem Delta columns

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VM Size (from What does “VM Size” mean in the Windows Task Manager?)

It's the total of all private (not shared) bytes allocated by this process, whether currently in physical memory or not.

This is the portion of the virtual address space a process allocates that is not shared with any other process. (It has nothing to do with the the physical, or RAM, usage.) Processes also allocate shared v.a.s.; shared v.a.s. is not included in this counter.

Further reading material: An introductory guide to Windows Memory Management

Page Fault (from Wikipedia)

A page fault is a trap to the software raised by the hardware when a program accesses a page that is mapped in the virtual address space, but not loaded in physical memory.

This is saying that whenever a program accesses something that is not in the real physical memory, a page fault occurs. The page has to be loaded from "backing store" (the pagefile, or for shared pages, a mapped file), which takes a very long time compared to pages that don't incur a page fault. HOWEVER: There are also "soft" or "in-memory" page faults. Windows maintains a cache (the standby page list + modified page list) of RAM pages that are not in any process working set (and so will incur a page fault if accessed) but have not yet been assigned to some other use. Page faults to these pages can be resolved without disk I/O. This still takes longer than if no page fault had occurred but is far faster than a "hard" page fault (one that requires disk reads).

Page faults are very common because the RAM can't keep everything. And in most workloads, most of them are "soft" page faults, which are resolved very quickly.

Mem Delta (from Microsoft TechNet)

The change in memory use, in kilobytes, since the last update. Unlike System Monitor, Task Manager displays negative values.

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