On 2/26/07, Nick Piggin <[EMAIL PROTECTED]> 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
transactions.png
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idle.png
Description: PNG image