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    SubjectRe: SMP performance degradation with sysbench
    Nish Aravamudan wrote:
    > 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?)

    I don't think it is exactly a matter of processes >= cores, but rather
    just a general problem at higher concurrency.

    --
    SUSE Labs, Novell Inc.
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