linux-kernel - Re: [rfc][patch] select_idle_sibling() inducing bouncing on westmere

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Date:	Sun, 27 May 2012 16:32:02 +0200
From:	Mike Galbraith <efault@....de>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Paul Turner <pjt@...gle.com>,
	Andreas Herrmann <andreas.herrmann3@....com>
Subject: Re: [rfc][patch] select_idle_sibling() inducing bouncing on westmere

On Sun, 2012-05-27 at 16:29 +0200, Mike Galbraith wrote: 
> On Sun, 2012-05-27 at 07:11 -0700, Arjan van de Ven wrote: 
> > On 5/27/2012 2:17 AM, Mike Galbraith wrote:
> > > On Sat, 2012-05-26 at 10:27 +0200, Mike Galbraith wrote: 
> > >> Hohum, back to finding out what happened to cpufreq.
> > > 
> > > Answer: nothing.. in mainline.
> > > 
> > > I test performance habitually, so just never noticed how bad ondemand
> > > sucks.  In enterprise, I found the below, explaining why cores crank up
> > > fine there, but not in mainline.  Somebody thumped ondemand properly on
> > > it's pointy head.
> > > 
> > > But, check out the numbers below this, and you can see just how horrible
> > > bouncing is when you add governor latency _on top_ of it. 
> > 
> > part of it is not ondemand, but cpufreq.
> > cpufreq forces you to schedule a kernel thread to change cpu
> > frequency... on the cpu that's already busy.
> > God knows what the scehduler then does in terms of load balancing.

(and yeah, I am lumping ondemand and cpufreq together, which is wrong)

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