linux-kernel - Re: 2.6.25.x: Wrong CPU frequency (cpufreq table) with p4-clockmod

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Date:	Sun, 01 Jun 2008 21:36:58 -0500
From:	Roger Heflin <rogerheflin@...il.com>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	Guntsche Michael <mike@...loops.com>, linux-kernel@...r.kernel.org
Subject: Re: 2.6.25.x: Wrong CPU frequency (cpufreq table) with p4-clockmod

Arjan van de Ven wrote:
> On Sun, 1 Jun 2008 23:57:14 +0200
> Guntsche Michael <mike@...loops.com> wrote:
> 
>> On May 30, 2008, at 16:07, Guntsche Michael wrote:
>>
>>> I am most interested in keeping the temperature of my CPU down,  
>>> which means slower fans, which means less noise.
>>> The main "problem" I have is that I do not know if this is a
>>> simple display issue or if I am having a more fundamental problem
>>> here.
>> Just FYI I found out the cause of my problem.
>>
>> Reverting
>> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=ed9cbcd40004904dbe61ccc16d6106a7de38c998
>> this patch resultsin the correct numbers for me.
>>
>> <cpuinfo snip>
>> processor	: 0
>> vendor_id	: GenuineIntel
>> cpu family	: 15
>> model		: 1
>> model name	: Intel(R) Pentium(R) 4 CPU 1.70GHz
>> stepping	: 2
>> cpu MHz		: 637.500
>> <snip>
>>
>> This patch was reverted because other people seem to have problems  
>> with it, reverting the revert does not look like a good idea either.
>> But since I know now, that this is just a "display" problem and  
>> everything is working otherwise, I'll just patch this locally for my  
>> machine here.
>>
> 
> just as a side note.. you do realize that with p4-clockmod, your cpu is
> still running at 1.7 GHz right? (it's just doing less work '-)

 From my testing, I believe the only thing that p4-clockmod does is forces an 
idle call when it could otherwise do work on an active process, so fullspeed and 
idle uses *EXACTLY* the same amount of power as p4-clockmod slower speed and 
idle (and therefore generates exactly the same amount of heat), the only power 
difference would be that if you were using p4-clockmod to slow down the cpu when 
it had an active running process (force the cpu to be idle a lot of the time 
even though it has work).   Arjan's point is that if you are using  p4-clockmod 
to slow down an idle cpu in hopes of saving power when the cpu is not being 
used, then it is not going to make *ANY* difference in the power usage at all.

I tried it on my p4 here, and cannot see any power difference in idle/fullspeed 
and idle/slowspeed, this is unlike the later power saving stuff that actually 
does slow down the cpu frequency, and you can measure a different amount of 
power usage with the different clock speeds and an idle cpu.

                                    Roger
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