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I'm having issues with CPU usage on a WinServ08 machine: Effectively, the CPU usage (and more noticeably on core 0 out of 3) is higher than I'd think, and task manager (when sorted by CPU %) shows only a few processes above 0 (and those don't add up to the reported CPU usage).

I'd really like to know why, is this just a rounding error (are all the other processes using 0.499% and just getting rounded down?) or is there another cause?

I'm completely baffled as the numbers just don't add up.

Specs: All drivers/software are up to date. Intel 2.83ghz Q9550 quad CPU+4gb of ram+Supermicro MBD-X7SBL-LN1-O motherboard

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What tool is reporting the CPU usage stats that you're using to compare against Task Manager? – WesleyDavid Jan 16 '10 at 4:31
Yes, although other tools (sysinternals process explorer) concur. I should clarify: Task manager gives a usage like 90% total, but I can only count up about 1/2 of that in the processes listed, making it hard to figure out what's going wrong. – cpf Jan 16 '10 at 4:33
Okay, it was a matter of the wording throwing me off. If I understand you correctly, Task Manager is showing high CPU usage in the "Performance" tab on the green graph, but the "Processes" tab when organized by CPU usage is not adding up to the same rough percentage seen on the Performance tab. Have you selected "Show processes from all users" on the Processes tab? – WesleyDavid Jan 16 '10 at 4:48
Yes, and show processes from all users is checked. – cpf Jan 16 '10 at 6:51
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Are you using a tool like process hacker that can separate userland CPU from kernel CPU? I think process explorer does the same thing as well.

Its possible most of the CPU usage is happening in kernel land.

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In process explored I saw that you could open system idle process, and inside "Deferred Procedure Calls" were taking a good amount of CPU. Not sure why that was lumped into "idle" CPU but that might be why the numbers don't add up. Thanks! – cpf Jan 16 '10 at 15:22
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It could be a rootkit issue. A rootkit could be hiding processes using a lot of CPU.

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