linux-kernel - Re: [RFC] sched: CPU topology try

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Message-ID: <20140108124253.GC31570@twins.programming.kicks-ass.net>
Date:	Wed, 8 Jan 2014 13:42:53 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...nel.org" <mingo@...nel.org>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"cmetcalf@...era.com" <cmetcalf@...era.com>,
	"tony.luck@...el.com" <tony.luck@...el.com>,
	"alex.shi@...aro.org" <alex.shi@...aro.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"corbet@....net" <corbet@....net>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"len.brown@...el.com" <len.brown@...el.com>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"amit.kucheria@...aro.org" <amit.kucheria@...aro.org>,
	"james.hogan@...tec.com" <james.hogan@...tec.com>,
	"schwidefsky@...ibm.com" <schwidefsky@...ibm.com>,
	"heiko.carstens@...ibm.com" <heiko.carstens@...ibm.com>
Subject: Re: [RFC] sched: CPU topology try

On Wed, Jan 08, 2014 at 12:35:34PM +0000, Morten Rasmussen wrote:
> > The harder case is where all 3 tasks are of equal weight; in which case
> > fairness would mandate we (slowly) rotate the tasks such that they all
> > get 2/3 time -- we also horribly fail at this :-)
> 
> I have encountered that one a number of times. All the middleware noise
> in Android sometimes give that effect.

You've got a typo there: s/middleware/muddleware/ :-)

> I'm not sure if the NUMA guy would like rotating scheduler though :-)

Hurmph ;-) But yes, the N+1 tasks on a N cpu system is rotten; any
static solution gets 2 tasks that run at 50%, any dynamic solution gets
the migration overhead issue.

So while the dynamic solution would indeed allow each task to (on
average) receive N/N+1 time -- a vast improvement over the 50% thing, it
doesn't come without down sides.
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