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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.

That means the whole size of memory a process allocates. It is the size of physical RAM plus the virtual memory size. Basically, the more VM Size a process allocates, the more resources it consumes. Processes with high VM size and small RAM size will often have page faults (see below).

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.

So basically, whenever a program requests something that is not in the real physical memory, a page fault occurs. The page has to be loaded from the virtual memory, which means longer loading time. To be more precise: Higher page fault values indicate poor performance because the virtual memory essentially is on the hard drive. As the hard drive has higher access times than RAM, the process needs to wait longer for the requested data.

Still page faults are very common because the RAM can't keep everything.

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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