Re: SMP performance degradation with sysbench

["Nish Aravamudan" <nish.aravamudan@gmail.com>]

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On 2/26/07, Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> Rik van Riel wrote:
> > Lorenzo Allegrucci wrote:
> >
> >> Hi lkml,
> >>
> >> according to the test below (sysbench) Linux seems to have scalability
> >> problems beyond 8 client threads:
> >> http://jeffr-tech.livejournal.com/6268.html#cutid1
> >> http://jeffr-tech.livejournal.com/5705.html
> >> Hardware is an 8-core amd64 system and jeffr seems willing to try more
> >> Linux versions on that machine.
> >> Anyway, is there anyone who can reproduce this?
> >
> >
> > I have reproduced it on a quad core test system.
> >
> > With 4 threads (on 4 cores) I get a high throughput, with
> > approximately 58% user time and 42% system time.
> >
> > With 8 threads (on 4 cores) I get way lower throughput,
> > with 37% user time, 29% system time 35% idle time!
> >
> > The maximum time taken per query also increases from
> > 0.0096s to 0.5273s. Ouch!
> >
> > I don't know if this is MySQL, glibc or Linux kernel,
> > but something strange is going on...
>
> Like you, I'm also seeing idle time start going up as threads increase.
>
> I initially thought this was a problem with the multiprocessor scheduler,
> because the pattern is exactly like some artificat in the load balancing.
>
> However, after looking at the stats, and testing a couple of things, I
> think it may not be after all.
>
> I've reproduced this on a 8-socket/16-way dual core Opteron. So far what
> I am seeing is that MySQL is having trouble putting enough load into the
> scheduler.

Here are some graphs from the 4-socket/8-way Xeon box (no SMT, no MC
in .config) I posted about earlier.

transactions.png resembles Nick's results pretty closely, in that a
drop-off occurs, at the same # of threads, too. That seems weird to
me, but I haven't thought about it too closely. Shouldn't Nick's be
dropping off closer to 16 threads (that would be 1 per core, then,
right?)

idle.png is the average % idle according to sar over each run from 1
to 32 threads. This appears to confirm what Rik was seeing.

Not sure if my data is hurting or helping, but this box remains
available for further tests.

Thanks,
Nish

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