Processor Scaling

Derek Broughton derek at
Wed Jan 7 01:20:35 UTC 2009

Ray Parrish wrote:

> Derek Broughton wrote:
>> Ray Parrish wrote:
>>> Well, as I said above I'm not worried about the extra power the "cpu"
>>> not the whole desktop takes... The cpu is just a small chip inside the
>>> box, and runs on 5 volts. Reading comprehension my friend...
>> Electricity comprehension my friend.  5V or 500V is (largely) irrelevant
>> - it's the power, not the voltage.
> Ok, I did some research, and this supports my view [From Wikipedia]
>> Dynamic frequency scaling reduces the number of instructions a
>> processor can issue in a given amount of time, thus reducing
>> performance. Hence, it is generally used when the workload is not
>> CPU-bound.
>> Dynamic frequency scaling by itself is rarely worthwhile as a way to
>> conserve switching power. Saving the most power requires dynamic
>> voltage scaling too, because of the V^2 component and the fact that
>> modern CPUs are strongly optimized for low power idle states. In most
>> constant-voltage cases it is more efficient to run briefly at peak
>> speed and stay in a deep idle state for longer (called "race to
>> idle"), than it is to run at a reduced clock rate for a long time and
>> only stay briefly in a light idle state. However, reducing voltage
>> along with clock rate can change those tradeoffs.

So what you've just said is that you didn't understand a word of what was 

Did you have a clue what it meant by V^2?

It also points out "modern CPUs are strongly optimized for low power idle 
states" - and you've yet to provide any evidence that yours _is_ one of 
those (it's not really even true - the _cutting edge_ cpus, are, but there 
are still huge numbers of commodity cpus that don't even do frequency 

Absolutely, it's right that voltage scaling is the best way to reduce power 
consumption - but that doesn't bear any relationship to what either you or I 
said above.

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