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

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Message-ID: <556F23E5.5020107@fb.com>
Date:	Wed, 3 Jun 2015 11:57:25 -0400
From:	Josef Bacik <jbacik@...com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	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 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.  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,

Josef

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