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Date: Tue, 4 Feb 2003 00:31:05 +0200
From: Ville Herva <vherva@niksula.hut.fi>
To: "Grover, Andrew" <andrew.grover@intel.com>
Cc: linux-kernel@vger.kernel.org
Subject: Re: CPU throttling??
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On Mon, Feb 03, 2003 at 01:14:18PM -0800, you [Grover, Andrew] wrote:
> > From: Dave Jones [mailto:davej@codemonkey.org.uk] 
> > Valdis.Kletnieks@vt.edu wrote:
> > 
> >  > It's conceivable that a CPU halted at 1.2Gz takes less 
> > power than one
> >  > at 1.6Gz - anybody have any actual data on this?  
> > Alternately phrased,
> >  > does CPU throttling save power over and above what the halt does?
> > 
> > Given that most decent implementations scale voltage as well as
> > frequency, yes, a lower speed will save more power.
> 
> You save the most power when the CPU is at the lowest voltage level, and
> in the deepest CPU sleep state (aka CPU C state).
> 
> Throttling offers a linear power/perf tradeoff if your system doesn't
> have C state support (or if you aren't using it) but really it is
> preferable to keep the CPU at its nominal speed, get the work done
> sooner, and start sleeping right away. The quote above makes it sound
> like the voltage is scaled when throttling, and that isn't accurate -
> voltage is scaled when sleeping (to counteract leakage current), at
> least on modern Intel mobile processors.

Interesting.

So, what sw does one need for this CPU C state? Which kernels support it /
which patches are needed? 2.5 only?

Also, which CPUs support it? Am I out of luck with my measly 1.4 Celeron
Tualatin?

So far I've only been doing "Make CPU Idle calls when idle", which I gather
is far from optimal?


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