Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE_WAKE

From	Josef Bacik <jbacik@fb.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for BALANCE_WAKE
Date	2015-06-03 18:00 +0200
Message-ID	<pxjfc-5DK-21@gated-at.bofh.it> (permalink)
References	(5 earlier) <pwEOK-4uj-7@gated-at.bofh.it> <pwY14-7M5-23@gated-at.bofh.it> <pxhGp-3tO-9@gated-at.bofh.it> <pxhQ5-3FJ-1@gated-at.bofh.it> <pxiVQ-5gl-9@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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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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Thread

Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for  BALANCE_WAKE Mike Galbraith <umgwanakikbuti@gmail.com> - 2015-06-03 17:40 +0200
  Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for  BALANCE_WAKE Josef Bacik <jbacik@fb.com> - 2015-06-03 18:00 +0200
    Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for  BALANCE_WAKE Mike Galbraith <umgwanakikbuti@gmail.com> - 2015-06-03 19:00 +0200
      Re: [PATCH RESEND] sched: prefer an idle cpu vs an idle sibling for  BALANCE_WAKE Josef Bacik <jbacik@fb.com> - 2015-06-03 19:20 +0200

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