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Date: Mon, 3 Feb 2003 21:18:06 +0000
From: Dave Jones <davej@codemonkey.org.uk>
To: "Grover, Andrew" <andrew.grover@intel.com>
Cc: Valdis.Kletnieks@vt.edu, John Bradford <john@grabjohn.com>,
       Seamus <assembly@gofree.indigo.ie>, linux-kernel@vger.kernel.org
Subject: Re: CPU throttling??
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On Mon, Feb 03, 2003 at 01:14:18PM -0800, Grover, Andrew wrote:

 > 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.

Most (all?[1]) other modern x86 mobile processors behave the way I mentioned.
AMD Powernow (K6 and K7), VIA longhaul/powersaver all have optimal voltages
they can be run at when clocked to different speeds. By way of example, a table from
my mobile athlon..

    FID: 0x12 (4.0x [532MHz])   VID: 0x13 (1.200V)
    FID: 0x4 (5.0x [665MHz])    VID: 0x13 (1.200V)
    FID: 0x6 (6.0x [798MHz])    VID: 0x13 (1.200V)
    FID: 0xa (8.0x [1064MHz])   VID: 0xd (1.350V)
    FID: 0xf (10.5x [1396MHz])  VID: 0x9 (1.550V)

Sure I *could* run that at 523MHz and still pump 1.550V into it,
but why would I want to do that ?

		Dave

[1] Unsure about the crusoe.

-- 
| Dave Jones.        http://www.codemonkey.org.uk
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