linux-kernel - Re: a question about p4_clockmod module on Xeon quad core processors

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Message-ID: <20090129172406.GB14096@redhat.com>
Date:	Thu, 29 Jan 2009 12:24:06 -0500
From:	Dave Jones <davej@...hat.com>
To:	Michael Tokarev <mjt@....msk.ru>
Cc:	Xiaoning Ding <dingxn@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: a question about p4_clockmod module on Xeon quad core
	processors

On Thu, Jan 29, 2009 at 03:27:24PM +0300, Michael Tokarev wrote:

 > > p4-clockmod doesn't change the processor frequency.
 > 
 > Hmm..  But what it really does, then?

it modulates the clock so the CPU isn't always availble for doing work.

 > I used it one one machine which had a flaky CPU cooler,
 > to reduce power consumption during hot summer days.

it reduces the amount of heat generated, by making jobs
take longer to finish. This comes at a cost of the CPU not
being able to enter lower C states for longer periods of time,
so you may end up actually using *more* power.

 > It worked, and reportedly the frequency varied from
 > 200MHz to the max of 2.4GHz (it's a P4 Xeon).

The CPU was always at 2.4GHz, even if it said '200MHz'.
It was just only doing work once in every 12 clocks.

	Dave

-- 
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