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:
>> 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
Virtually all of the sleep time is coming from unix_stream_recvmsg, which
seems to be what the clients and server threads use to communicate with.
There doesn't seem to be any other tell-tale event that the database is
It seems like it might at least partially be a problem with MySQL
I found a couple of interesting issues so far. Firstly, the MySQL version
that I'm using (5.0.26-Max) is making lots of calls to sched_setscheduler
attempting to fiddle with SCHED_OTHER priority in what looks like an
attempt to boot CPU time while holding some resource. All these calls
actually fail, because you cannot change SCHED_OTHER priority like that.
Adding a hack to make it fall through to set_user_nice provides a boost
which eliminates the cliff (but a downward degredation is still there).
Secondly, I've raised the thread numbers from 16 to 32 for my system,
which also provides a bit more (although doesn't help the downward
Combined, it looks like around 30-40% improvement past 16 threads. It
isn't anything like making up for the dropoff seen in the blog link, but
different systems, different mysql version... I wonder how close we are
with this hack in place?
Attached is a graph of my numbers, from 1 to 32 clients. plain = 126.96.36.199,
sched is with the attached sched patch, and thread is with 32 rather than
Anyway, I'll keep experimenting. If anyone from MySQL wants to help look
at this, send me a mail (eg. especially with the sched_setscheduler issue,
you might be able to do something better).
SUSE Labs, Novell Inc.