Measuring 30b CPU speed reduction

[Claudio L.]

(08-09-2015 11:52 AM)Thomas Radtke Wrote:  

(08-03-2015 05:05 AM)cyrille de brébisson Wrote:  [...] (Car analogy is incorrect)...

This reduces the power consumption by a factor much greater than the ~5 time drop in CPU speed [...]

So, the correct car analogy is to say that driving a car at 20Mph DOES save gaz compared with driving it at 200Mph

This is math from a different universe. 200/5 = 40 or 100/5 = 20. Close to what Csaba got and hence my analogy was:

Quote:[...] a car reducing its speed from 100 to 18 km/h

I meant no offense. Just didn't knew the underclocked CPU was more efficient.

Have I missed something?

Well, the optimal gas consumption in a car is actually between 90 km/h and 120 km/h, so driving at 18 km/h you will use much more gas than driving at 100 km/h. So the analogy was indeed incorrect.
The statement would be correct, however, for much higher speeds, where the quadratic wind resistance forces you to use more gas, hence he went from 100 km/h to 200 mph in the example. Of course, you are correct he should've kept the same increase factor.

On a CPU, power is more or less linear with the clock frequency, while the speedup is less than linear (due to those wait states to access ram/flash) but very close, so in general using a higher clock frequency will consume about the same or slightly more total energy (this is energy, not power, as power is higher but for a shorter time). That's assuming nothing else changes (in this case the battery's internal resistance changes and this is no longer true).