linux-kernel - Re: PROBLEM: Celeron Core

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Date:	Sun, 20 Jan 2008 13:24:15 -0600
From:	Robert Hancock <hancockr@...w.ca>
To:	Matt Mackall <mpm@...enic.com>
Cc:	David Newall <davidn@...idnewall.com>,
	Andi Kleen <andi@...stfloor.org>,
	Chodorenko Michail <misha@....by>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: PROBLEM: Celeron Core

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.
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