linux-kernel - Re: SMP performance degradation with sysbench

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Date:	Tue, 27 Feb 2007 18:52:30 -0800
From:	"Nish Aravamudan" <nish.aravamudan@...il.com>
To:	"Bill Davidsen" <davidsen@....com>
Cc:	"Paulo Marques" <pmarques@...popie.com>,
	"Rik van Riel" <riel@...hat.com>,
	"\"J.A. Magallón\"" <jamagallon@....com>,
	"Hiro Yoshioka" <hyoshiok@...aclelinux.com>, davej@...hat.com,
	harlan@...select.com, nickpiggin@...oo.com.au,
	l_allegrucci@...oo.it, linux-kernel@...r.kernel.org, mingo@...e.hu,
	suparna@...ibm.com, jens.axboe@...cle.com
Subject: Re: SMP performance degradation with sysbench

On 2/27/07, Bill Davidsen <davidsen@....com> wrote:
> Paulo Marques wrote:
> > Rik van Riel wrote:
> >> J.A. Magallón wrote:
> >>> [...]
> >>> Its the same to answer 4+4 queries than 8 at half the speed, isn't it ?
> >>
> >> That still doesn't fix the potential Linux problem that this
> >> benchmark identified.
> >>
> >> To clarify: I don't care as much about MySQL performance as
> >> I care about identifying and fixing this potential bug in
> >> Linux.
> >
> > IIRC a long time ago there was a change in the scheduler to prevent a
> > low prio task running on a sibling of a hyperthreaded processor to slow
> > down a higher prio task on another sibling of the same processor.
> >
> > Basically the scheduler would put the low prio task to sleep during an
> > adequate task slice to allow the other sibling to run at full speed for
> > a while.
> >
> > I don't know the scheduler code well enough, but comments like this one
> > make me think that the change is still in place:
> >
> >>     /*
> >>      * If an SMT sibling task has been put to sleep for priority
> >>      * reasons reschedule the idle task to see if it can now run.
> >>      */
> >>     if (rq->nr_running) {
> >>         resched_task(rq->idle);
> >>         ret = 1;
> >>     }
> >
> > If that is the case, turning off CONFIG_SCHED_SMT would solve the problem.
> >
> That may be the case, but in my opinion if this helps it doesn't "solve"
> the problem, because the real problem is that a process which is not on
> a HT is being treated as if it were.
>
> Note that Intel does make multicore HT processors, and hopefully when
> this code works as intended it will result in more total throughput. My
> supposition is that it currently is NOT working as intended, since
> disabling SMT scheduling is reported to help.

It does help, but we still drop off, clearly. Also, that's my
baseline, so I'm not able to reproduce the *sharp* dropoff from the
blog post yet.

> A test with MC on and SMT off would be informative for where to look next.

I'm rebooting my box with 2.6.20.1 and exactly this setup now.

Thanks,
Nish
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