Messages in this thread | | | Date | Wed, 28 Feb 2007 13:22:56 +1100 | From | Nick Piggin <> | Subject | Re: SMP performance degradation with sysbench |
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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.
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