Messages in this thread | | | Date | Tue, 27 Feb 2007 18:51:06 -0800 | From | "Nish Aravamudan" <> | Subject | Re: SMP performance degradation with sysbench |
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On 2/27/07, Nick Piggin <nickpiggin@yahoo.com.au> wrote: > 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.
Ok, thanks for the clarification.
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