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I'm trying to monitor total CPU usage on a Win2k8 R2 server using Performance Monitor.

I'm running in HyperV VM on a 12-core system but only a single hyper-threaded core is allocated to this image (so two CPUs from the OS perspective).

I'm looking at the 'Report' view of Current Activity, with a report showing the following data points:

   Processor(_Total)\%Idle Time
   Processor(_Total)\%Processor Time

Processor(_Total)\'%Processor Time' regularly jumps up to values between 15% - 30% and then falls back to '---' and then to '0.000', even when the server is apparently idle.

Processor(_Total)\'% Idle Time' however remains at or near 100.00 most of the time the '%Processor Time' values are jumping around, but will drop below 100 if I do something that I know is producing load like running some SQL queries.

This had lead me to believe that '% Idle Time' is a more "reliable" measure of true CPU load than the "%Processor Time", but after much searching I can't figure out why this would be the case?

Now what's really freaky is if I add 'Processor Information(0,_Total)\%Idle Time' into the same report, the 'Processor(_Total)\%Processor Time' and 'Processor(_Total)\%Idle Time' all of a sudden start showing consistent and believable data, while the 'Processor Information(0,_Total)\%Idle Time' value jumps around in strange ways and even shows values > 100%.

Sorry for the long-winded question... this is really strange behavior, has anyone seen it before?

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When it comes to monitoring CPU usage inside a VM, all bets are off – Mark Henderson Jul 14 '11 at 8:33

I've never actually witnessed this and it does indeed look strange, but I'd like to point out that on virtualized systems CPU statistics are well known to be somewhat unreliable; after all, the VM execution can and will be suspended or slowed down while physical CPU time gets allocated to other guest systems.

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