`Matt Mackall wrote:> Your usage of "overall power" here is wrong. Power is an instantaneous> quantity (1/s) like velocity, and you are comparing it to energy which> is not an instaneous quantity, more like distance.> > If we throttle the velocity of a car from 100km/h to 50km/h, it'll> obviously take longer for it travel a given distance. Now what will it> mean when we ask about its "overall velocity" when it reaches its> destination? We surely don't mean the distance travelled - that's not a> velocity! We can perhaps talk about its average velocity, which will> obviously be smaller.You are right.. it should be that overall energy usage is higher with clock throttling.> >> Real CPU clock throttling schemes like SpeedStep, PowerNow, etc. >> actually do increase performance per watt when they kick in.> > That may be true. But the statement "throttling does not reduce power> usage" remains false. And the statement "throttling reduces heat> production but not power usage" remains physically impossible.It reduces the rate of power usage (watts), however it will likely not decreate or even increase the energy usage (i.e. watt-hours) of any given computational task.> > It might be true that "throttling increases energy usage per unit of> computation relative to no power saving measures at all", but that is> not incompatible with "throttling lets you run your laptop on battery> longer than no power saving measures at all", which is often what people> care about.> > Voltage/frequency reduction is obviously a much better solution if it's> available as reducing voltage reduces power usage quadratically rather> than linearly. But beyond the quadratic/linear thing, the concept is the> same: use less power and your battery lasts longer.Clock throttling is not likely to save your battery, unless you have tasks that are running at 100% CPU for an unlimited time or something, and you force your CPU to throttle. Normally most people have tasks that run and then the CPU idles - loading an email, displaying a web page, etc. Clock throttling will just make these tasks utilize the CPU for a longer time proportional to the amount clock throttling and therefore negate any gains in battery usage.`