Re: SMP performance degradation with sysbench

[Nish Aravamudan]

On 2/27/07, Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
> Nish Aravamudan wrote:
> > On 2/26/07, Nick Piggin <nickpiggin@xxxxxxxxxxxx> 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?)
>
> I don't think it is exactly a matter of processes >= cores, but rather
> just a general problem at higher concurrency.

Ok, thanks for the clarification.

-Nish
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