linux-kernel - Re: [RFC PATCH v1 0/3] Saving power by cpu evacuation using sched

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Date:	Tue, 28 Apr 2009 10:52:37 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	svaidy@...ux.vnet.ibm.com,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Suresh B Siddha <suresh.b.siddha@...el.com>,
	Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vatsa <vatsa@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Andi Kleen <andi@...stfloor.org>,
	Gregory Haskins <gregory.haskins@...il.com>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arun Bharadwaj <arun@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH v1 0/3] Saving power by cpu evacuation using
	sched_mc=n


* Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:

> > > Also, the user interface should be that single thermal 
> > > capacity knob, more fine grained control is undesired.
> > 
> > For power savings, a single evacuation knob will do.  While for 
> > thermal we will need additional parameters to choose the right 
> > cores to evacuate.  Some sort of directional/vector parameter.
> 
> Why? are machines that non-uniform in cooling capacity that it 
> really matters which core generates the heat? Sounds like badly 
> designed hardware to me.
> 
> I would expect it to only be the total head generated/power taken 
> from the rack unit.

If we add thermal throttling at the kernel level then a single knob 
(with a percentile-ish unit) is probably the furthest we will go - 
with "not doing it at all" still being the other, very tempting 
alternative.

If the only technical way you can find to do it is via myriads of 
non-intuitive knobs and per core settings - then the answer is 
really 'no thanks'.

	Ingo
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