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I have a question regarding task manager in Windows 8. It shows us CPU utilization and speed. From what I understand, utilization is how much of the processor time is spent running processes. Speed is what the current clock speed of the processor is (it could be dynamically changing due to power saving features). My question is, say I have a computer that idles at 0.7 GHz clock speed and runs at around 8% utilization average. If I disable power saving features and let the computer idle at its max capacity of 2.0 GHz, shouldn't the utilization when idle average to a much smaller number?

That is not the case with my current laptop; whether my computer is running at 0.7GHz average or 2.0 GHz average, utilization always falls around 8%.

Could someone explain this to me please?

Thank you!

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2 Answers

up vote 3 down vote accepted

The lack of difference in utilization with different clock frequencies might be due the computation not being limited by clock speed. E.g., if memory access latency or bandwidth is the main factor limiting performance, then decreasing clock frequency may not significantly reduce performance (so utilization would remain more or less constant).

Another factor might be the granularity of tracking utilization. If simple 1ms timing is used, then any fraction of the time granule could be counted as an entire granule. If the activity is frequent (80 times per second) but extremely short-lived (<1ms for each burst of activity in full speed mode--even just 500,000 CPU cycles [0.7ms at 0.7GHz] can accomplish some work), then both clock frequencies would have the same measured utilization.

It is also possible that in low-power mode the system is doing less work. This could be a very reasonable design choice. Extra work in full speed mode might allow greater responsiveness or provide some other benefit at the cost of energy efficiency. In low-power mode, energy efficiency would be more aggressively sought.

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Let me explain by analogy: If I work on an assembly line and 8% of my time is taken up screwing a screw in to the machine, if I work twice as fast it still takes 8% of my total time to screw in the screw, it is just my total time is now half as long.

So your processor speed increased but whatever activity was using the CPU still uses the same ratio of the CPU, it is just doing more of it per second.

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