Puzzling difference between PF and Mem Usage?

November 29, 2010 at 00:38:57
Specs: Windows XP Pro, Quad Q9450/4GB
As part of monitoring and detective work to track down some obscure crashes, I've been watching the 'PFUsage' number under the Performance tab of Task Manager in XP Pro.

One thing that puzzles me is why there's such a large difference between that number and
the total of the 'Mem Usage' column under the Processes tab?

Right now for example, PF usage = 918 MB, while Mem usage = 505 MB.(I copied the data into Excel to get that sum.)

I'm guessing this has an obvious explanation?

Terry, East Grinstead, UK

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November 29, 2010 at 05:08:39
Microsoft has a long history of using misleading or confusing labels in Task Manager. This is one of the worst examples but it is by no means the only one. Possibly a misguided attempt to make things easier to understand. In fact it has caused endless confusion, even among computer professionals.

Adding up all the numbers in the memory usage column will rarely provide meaningful results.

"PF Usage" isn't actual pagefile usage at all. It is really the same as the commit charge but displayed in different units. The commit charge is rather difficult to explain so I will not go into it in any detail. It is NOT memory usage, pagefile usage, or any combination of the two. It can be thought of as potential pagefile usage. It is the sum of everything in the pagefile plus everything in RAM that could be copied to the pagefile. The commit charge can be, and often is, larger then RAM size. This does not indicate any kind of problem and is not a reliable indicator of impaired performance.

The only thing you really need to know about the commit charge is that it should be kept well below the commit limit. The commit limit is the sum of pagefile size, plus RAM size, minus some overhead. It is of little performance as a performance indicator. The "Available" memory is of far greater importance.

Interpreting Task Manager displays isn't nearly as simple or straightforward as it might appear.

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November 29, 2010 at 08:01:40
Many thanks, much appreciated. Although mostly beyond my technical grasp.

In 'lay' terms I'm looking for some reasonabaly simple, preferably visual way to detect if my PC is 'running out of memory'. This is just one possible cause I'm exploring for the intermittent but exasperating crashes I've been getting.

(Others include a damaged or overheating nVidia 880GT graphics card, a so far undetected virus/trohan/whatever, scambled drivers, a flakey application such as the latest version of my video editing program, etc).

Are you saying that TM doesn't offer such a visual tool? How about Process Explore, which I also have installed? Or any other suggestions?

Terry, East Grinstead, UK

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