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On Windows XP SP3, in Task Manager I assume that the "System Idle Process" is not counted as part of the overal "CPU Usage", therefore at any time the sum of both should be 100%, which seems to be true most of the time.

However sometimes, the CPU Usage may be at 50% or more while the idle process is still using 90% or more! For example when I start a program on my dual-core, I see one CPU going to 100% and the other staying fairly low, so the overall usage is around 60%, but the idle process is still above 90%.

What could be the reason for this discrepancy, and what is my CPU really doing at that time?

Note: I've checked the "Show processes from all users" box in the Processes tab.

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up vote 2 down vote accepted

XP Task manager is not a very good tool for looking at this kind of information, that is why MS added Resource Monitor to Vista and W7, a much better monitoring tool, task manager was also improved. XP does have Performance Monitor, type perfmon into a run box, still not anything like Vista-W7 tools

Some tools you can add to XP


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This answer doesn't help. All the tools you mention are looking at the same set of counters; they are just different displays. – Jamie Hanrahan Aug 2 '15 at 20:01
Evidently the person that asked the question thought it was useful and accepted my answer. – Moab Aug 3 '15 at 22:16
Nevertheless, my statement holds. These tools will all show you the same numbers because they're all using the same data sources. – Jamie Hanrahan Aug 3 '15 at 22:21
Ok so you get a blue ribbon for commenting on a 5+ year old accepted answer, now move on. – Moab Aug 3 '15 at 22:47

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