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

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Date:	Sun, 27 May 2012 16:29:15 +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 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.

Well, it'll take a spot that could have been used to authorize an affine
wakeup for one, switch freqs a tad too late if it doesn't preempt, not
to mention munching valuable cycles.

> (yes this is one of the things that will be fixed in the code that we
> now have working internally, and we're now making sure does not regress)

Cool.

> btw, on modern Intel CPUs, where in Idle, you have a frequency of Zero,
> regardless of what you ask for when you're running, and where an idle
> cpu is clock gated.... the performance governor behaves almost the same
> as ondemand in terms of power (due to the suckitude of ondemand)... but
> much better performance.

That sounds like it should rock.

-Mike

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