Messages in this thread | | | Date | Sun, 26 Feb 2006 21:55:13 +0100 | From | Dominik Brodowski <> | Subject | Re: Status of X86_P4_CLOCKMOD? |
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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
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