Re: SMP performance degradation with sysbench

[Nick Piggin <nickpiggin@yahoo.com.au>]

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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