Re: Status of X86_P4_CLOCKMOD?

[Pavel Machek <pavel@suse.cz>]

Hi!

> > > Not in this case no.
> > >...
> > 
> > Sorry for the dumb question, but how could this work physically?
> > 
> > If a computer produces less heat with the same power consumption, what 
> > happens with the other energy?
> 
> No. Let's do the math (again), and (again) for the actual values of an Intel
> Pentium(R) M Processor, 1400 MHz @ 1.484 V, even though the same rules of
> physics, logic and mathematics apply to _all_ processors.
> 
> Power consumption in idle state C2 (Stop-Grant state)	 7.3 W
> Power consumption when "skipping instructions"
> 	because of throttling (Stop-Grant state)	 7.3 W
> 
> Power consumption when doing work			22.0 W
> 
> 
> This means that if the processor idle percentage is _larger_ than (1 -
> throttling rate), throttling has no effect at all.
> 
> 
> Now, let's assume there is some work for the CPU to do which keeps it busy
> for one second @ 1.4 GHz. How much energy is needed to get this work done?
> 
> 0% throttling:		22 Ws	(1s)
> 25% throttling:		24 Ws	(1.3s)
> 50% throttling:		29 Ws	(2s)
> 75% throttling:		44 Ws	(4s)
> 
> 
> Now let's also assume there is nothing else to do during a span of four
> seconds: then, independent of the throttling setting, the CPU power
> consumption is 44 Ws for these four seconds.
> 
> 
> However: for the 75% throttling state, the CPU only produces 11 W of heat
> _all the time_ -- this means, the fan or air conditioning must only consider
> 11 W. For 0%, the CPU may produce 44 W of heat in a second -- and to cool
> that sufficiently, the fan _may_ need to run faster, which consumes more
> energy than is saved by only having to cool 7.3 W (instead of 11W) the other
> three seconds.
> 
> 
> So: P4-clockmod style throttling only makes sense if either
> 
> a) the idle handler does not enter the Stop-Grant state (C2) efficiently, or
> 
> b) the load varies significantly over time in a manner which has effect on
>    the fan, and where the latency induced by throttling doesn't matter.

c) your load is something like game of doom, where you want
  to play for half an hour with least power consumed. You
  don't really need 1000 fps, 100 fps is enough.
						Pavel
-- 
Thanks, Sharp!
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