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 19:43:31 +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 13:16 -0400, Josef Bacik wrote:

> Eesh ok, do you happen to remember how you ran tbench so I can add it to 
> my tests here?  In addition to fixing this problem we're also interested 
> in tracking performance of new kernels so we don't have to do this "what 
> the hell went wrong in the last 6 releases" dance every year, so I'm 
> throwing every performance thing we find useful in our test 
> infrastructure.  Thanks,
> 
> Josef

Start a tbench server, then tbench -t 30 1 localhost.  You're unlikely
to find anything as painful as that bouncing cow bug was, but you won't
have to look hard at all to find bounce pain.

There are also other loads like your server where waking to an idle cpu
dominates all else, pgbench is one of those.  In that case, you've got a
1:N waker/wakee relationship, and what matters above ALL else is when
the mother of all work (the single server thread) wants a CPU, it had
better get it NOW, else the load stalls.  Likewise, 'mom' being
preempted hurts truckloads.  Perhaps your server has a similar thing
going on, keeping wakees the hell away from the waker rules all.

	-Mike

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