linux-kernel - Re: [RFC PATCH v2 0/2] Saving power by cpu evacuation sched_max_capacity

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Date:	Thu, 14 May 2009 20:28:29 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	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>,
	Ingo Molnar <mingo@...e.hu>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vatsa <vatsa@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	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 v2 0/2] Saving power by cpu evacuation
	sched_max_capacity_pct=n

* Andi Kleen <andi@...stfloor.org> [2009-05-13 17:10:54]:

> > > Yes that's fine and common, but you actually need to save power for this,
> > > which throttling doesn't do.
> > > 
> > > My understanding this work is a extension of the existing
> > > sched_mc_power_savings features that tries to be optionally more 
> > > aggressive to keep complete package idle so that package level
> > > power saving kicks in.
> > > 
> > > I'm just requesting that they don't call that throttling.
> > 
> > Ah no, this work differs in that regard in that it actually 'generates'
> > idle time, instead of optimizing idle time.
> 
> That is what i meant with "more aggressive to keep complete packages idle"
> above.

Hi Andi,

There is a difference in the framework as Peter has mentioned, we are
trying to create idle times by forcefully reducing work.  From an
end-user point of view, this can be seen as a logical extension of
sched_mc_power_savings... v1 of the RFC extends the framework.

However Ingo suggested that the knob is not intuitive and hence I have
tried to switch to a percentage knob sched_max_capacity_pct.

I am interested in an easy, simple and intuitive framework to evacuate
cores which may imply forcefully reducing (throttling) work.
 
> > Therefore it takes actual cpu time away from real work, which is
> > throttling. Granted, one could call it limiting or similar, but
> > throttling is a correct name.
> 
> That will be always ongoing confusion with the existing established
> term. 
> 
> If you really need to call it throttling use "scheduler throttling"
> or something like that, but a different word would be better.

I think 'scheduler throttling' is good so that we avoid the term 'CPU
throttling' or core throttling.  I had named this cpu evacuation or
core evacuation just to avoid confusion with hardware throttling.

--Vaidy
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