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Date:      Fri, 04 Mar 2005 08:55:02 -0800
From:      "Kevin Oberman" <oberman@es.net>
To:        Nate Lawson <nate@root.org>
Cc:        current@FreeBSD.ORG
Subject:   Re: patch: p4tcc and speedstep cpufreq drivers 
Message-ID:  <20050304165502.19EED5D07@ptavv.es.net>
In-Reply-To: Your message of "Thu, 03 Mar 2005 21:37:05 PST." <4227F401.7000204@root.org> 

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> Date: Thu, 03 Mar 2005 21:37:05 -0800
> From: Nate Lawson <nate@root.org>
> 
> Kevin Oberman wrote:
> > OK. This makes me feel a bit better, but I still think I'll leave TCC
> > out of the equation as it makes the various frequency steps vary uneven
> > to the point that lowering dev.cpu.0.freq would increase performance
> > (and the reverse, as well) and it causes my system to hang when
> > throttled back too far. It never hangs with TCC disabled although my
> > lowest "frequency" is now just 150 MHz.
> 
> Would you test with hint.acpi_throttle.0.disabled="1" instead of 
> disabling p4tcc?  I think p4tcc is not the problem, it's the combination 
> of the two.  I think there are some problems when both the chipset 
> (externally) and processor (internally) assert STOPCLOCK.  If this works 
> for you with no hangs, I'll commit code to disable acpi_throttle when 
> p4tcc is present.  p4tcc is more efficient than acpi_throttle since the 
> latter is done through the chipset, giving more chance for race 
> conditions, latency, etc.

Looks like you are right on the button. p4tcc with throttling disabled
yields the best results I have seen. The performance is just a
little better than the "normalized" value I would expect where
throttling produced performance just a little worse. As long as I don't
run both, I don't hang at any speed and I don't get increased
performance with decreased speed.

I really want to try some tests while actively monitoring current draw
some day, but it will require hacking on a power brick and I don't have
one I can play with at the moment. That would provide some REAL
indication of power savings with reduced performance and make tuning
more accurate.

I am appending the test results. As usual they are on a system running
single-user and are very consistently reproduceable with standard variation
of less than .4% and usually under .2%.
-- 
R. Kevin Oberman, Network Engineer
Energy Sciences Network (ESnet)
Ernest O. Lawrence Berkeley National Laboratory (Berkeley Lab)
E-mail: oberman@es.net			Phone: +1 510 486-8634

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