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Why does a CPU consume different quantities of power at 2Ghz when

  1. it is executing an instruction
  2. it is not executing any instruction

Should'nt the CPU be consuming same power irrespective of whether it is executing an instruction or not ? (since power is depends on frequency and not the execution of instruction)

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3 Answers 3

up vote 5 down vote accepted

Ah - no. Power does not only deend on frequency, it also depends on what the transistors do. So, yes, power usage even WHILE executing instructions can fluctuate - especially given that with out of order execution and parallelism inmodern CPUs the number of instructions executed can vary depending on what instructions are executed.

The fact that CPU power usage varies EVEN AT THE SAME FREQUENCY was already observes long time ago before cpu power management even became a factor.

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thanks tomtom.. that was an interesting piece of info .. I have one more question . Can CPUs be in C1 state while running at 2 Ghz ? Is'nt the frequency scaled down when the CPU is in C1 state ? (once HLT is executed) ? –  kashyapa Oct 31 '10 at 8:34
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That depends on the CPU. Newer CPUs - if the OS supports it - can even turn off complete cores. It is a decision by the designers - and not an easy one. –  TomTom Oct 31 '10 at 8:41

You can talk about frequency and CPU parallelism and anything else you like but at the end of the day a CPU is an electronic component and like any other electronic component it consumes power and it consumes more power when it's performing work then when it's not.

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This was explained in - http://serverfault.com/questions/196308/how-does-a-power-graph-of-a-server-running-a-job-look-like-when-there-is-no-cpu-p

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@user48838 : That does not answer my question - why should it consume more power while running an instruction and less power while not running any instruction ? (assume CPU is at constant frequency always) –  kashyapa Oct 31 '10 at 7:43
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But it is not with modern processors. At the electronics level, when processing takes place, there are actually more electrical transactions taking place as more gates are being activated in order to complete the computational requests/actions. –  user48838 Oct 31 '10 at 8:46
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Kashyapa - are you interested in the real world or are you talking about theory? Because in practice, as user48838 says, the clock speed of a modern processor is variable, with cores able to run faster or slower as workload permits. TomTom's given you a very good explanation of power variations at one speed but as well as that it's impossible to understand how modern processors manage power without understanding how and why their clockspeed changes too. –  RobM Oct 31 '10 at 9:07

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