linux-kernel - Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE

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Date:	Wed, 03 Jun 2015 18:53:06 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Josef Bacik <jbacik@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>, mingo@...hat.com,
	linux-kernel@...r.kernel.org, morten.rasmussen@....com,
	kernel-team <Kernel-team@...com>
Subject: Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for
 BALANCE_WAKE

On Wed, 2015-06-03 at 11:57 -0400, Josef Bacik wrote:
> On 06/03/2015 11:30 AM, Mike Galbraith wrote:
> > On Wed, 2015-06-03 at 16:24 +0200, Peter Zijlstra wrote:
> >> On Wed, 2015-06-03 at 10:12 -0400, Rik van Riel wrote:
> >>
> >>> There is a policy vs mechanism thing here. Ingo and Peter
> >>> are worried about the overhead in the mechanism of finding
> >>> an idle CPU.  Your measurements show that the policy of
> >>> finding an idle CPU is the correct one.
> >>
> >> For his workload; I'm sure I can find a workload where it hurts.
> >>
> >> In fact, I'm fairly sure Mike knows one from the top of his head, seeing
> >> how he's the one playing about trying to shrink that idle search :-)
> >
> > Like anything where scheduling latency doesn't heavily dominate.  Even
> > if searching were free, bounces aren't, even for the very light.
> >
> 
> If scheduling latency doesn't hurt then making the search shouldn't 
> matter should it?  I get that migrations aren't free, but it seems like 
> they can't hurt that much.

Nah, they don't hurt much :)

commit e0a79f529d5ba2507486d498b25da40911d95cf6
Author: Mike Galbraith <bitbucket@...ine.de>
Date:   Mon Jan 28 12:19:25 2013 +0100

    sched: Fix select_idle_sibling() bouncing cow syndrome
    
    If the previous CPU is cache affine and idle, select it.
    
    The current implementation simply traverses the sd_llc domain,
    taking the first idle CPU encountered, which walks buddy pairs
    hand in hand over the package, inflicting excruciating pain.
    
    1 tbench pair (worst case) in a 10 core + SMT package:
    
      pre   15.22 MB/sec 1 procs
      post 252.01 MB/sec 1 procs
    

>   This application is huge, it's our 
> webserver, we're doing like 400 requests per second on these things, and 
> hands down moving stuff to idle cpus is beating the pants off of staying 
> on the same cpu.  Is there a specific workload I could build a test for 
> that you think this approach would hurt?  Thanks,

Search cost hurts fast movers, as does dragging even a small footprint
all over the place, as you can see above.

	-Mike

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