Re: Status of X86_P4_CLOCKMOD?

[Dominik Brodowski <linux@dominikbrodowski.net>]

Hi,

On Sun, Feb 26, 2006 at 09:39:41PM +0100, Johannes Stezenbach wrote:
> > 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.
> 
> Do you have the numbers for a Pentium(R) 4 HT? (I couldn't find
> anything substantial with google.) Especially C2 vs. C2 + throttling?
> Because the way I remember having read somewhere, the idle
> (C2) power consumption of the P4 is significantly higher
> than with the Pentium(R) M.

Unfortunately, I do not have these numbers present. You can check the
processor specification sheets at Intel's website, though.


> > 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.
> 
> On a Pentium(R) M, but how about P4? The two have very different
> architectures, don't they?

They have different architectures, but again -- AFAIK -- throttling means STPCLK
is stopped, which is equivalent to the Stop-Grant state. And that's what's
usually entered in C2-type idle sleep.

> > 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.
> 
> This is all fine, but why would anyone use throttling when the
> CPU has work to do (except for thermal emergency throttling)?

That's exactly the opposite of what you should do, if idling works at least
reasonably well: only enter throttling if the CPU has some substancial
workload.

> > 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.
> 
> Maybe my previous mails were not clear enough: The goal is to
> reduce idle power consumption (and by that fan noise). The PC
> is running but is idle, e.g just listening for possible incoming
> jabber messages or whatever.

Most probably, the idle handler can't make use of the Stop-Grant state (C2)
here, so this is case a) noted above.

> p4-clockmod: has errata -- disabling frequencies lower than 2ghz


	Dominik
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