linux-kernel - Re: [PATCH] sched: wake-affine throttle

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Date:	Fri, 03 May 2013 08:14:36 +0200
From:	Mike Galbraith <efault@....de>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Peter Zijlstra <peterz@...radead.org>,
	Alex Shi <alex.shi@...el.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Turner <pjt@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Ram Pai <linuxram@...ibm.com>
Subject: Re: [PATCH] sched: wake-affine throttle

On Fri, 2013-05-03 at 13:57 +0800, Michael Wang wrote: 
> Hi, Mike
> 
> Thanks for your reply.
> 
> On 05/03/2013 01:01 PM, Mike Galbraith wrote:
> [snip]
> >>
> >> If this approach caused any concerns, please let me know ;-)
> > 
> > I wonder if throttling on failure is the way to go.  Note the minimal
> > gain for pgbench with the default 1ms throttle interval.  It's not very
> > effective out of the box for the load type it's targeted to help, and
> > people generally don't twiddle scheduler knobs.  If you throttle on
> > success, you directly restrict migration frequency without that being
> > affected by what other tasks are doing.  Seems that would be a bit more
> > effective.
> 
> This is a good timing to make some conclusion for this problem ;-)
> 
> Let's suppose when wake-affine failed, next time slice got a higher
> failure chance, then whether throttle on failure could be the question like:
> 
> 	throttle interval should cover more failure timing
> 	or more success timing?
> 
> Obviously we should cover more failure timing, since it's just wasting
> cycle and change nothing.
> 
> However, I used to concern about the damage of succeed wake-affine at
> that rapid, sure it also contain the benefit, but which one is bigger?
> 
> Now if we look at the RFC version which throttle on succeed, for
> pgbench, we could find that the default 1ms benefit is < 5%, while the
> current version which throttle on failure bring 7% at most.

OK, so scratch that thought.  Would still be good to find a dirt simple
dirt cheap way to increase effectiveness a bit, and eliminate the knob.
Until a better idea comes along, this helps pgbench some, and will also
help fast movers ala tbench on AMD, where select_idle_sibling() wasn't
particularly wonderful per my measurements.

-Mike

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