Re: Perf regression in 2.6.32 (Ubuntu 10.04 LTS)

Domas Mituzas wrote:
> I've been playing around today a lot with sysbench, and observed that 2.6.32 kernel supplied by Ubuntu is having perf regression with PG (which does not affect MySQL), compared to 2.6.28 builds I have.
> What I observed can be seen in a paste at http://p.defau.lt/?8_GQV82Pz3_SDZbNOdP93Q (db12 is 2.6.28, db20 is 2.6.32 - 2.6.32-24-server).
>
> Machines are two socket quad-opterons 2356s.
>
> oprofile output can be seen at http://p.defau.lt/?OIR1vDFK4cze_fmBTQbV9w - system has >20% of idle cpu, which is somewhere in the top symbol :)
>

Are you using the same filesystem setup on both setups? And regardless,
what is that filesystem? We know that between 2.6.28 and 2.6.32 the
kernel improved how it handles fsync requests in a good way from a
reliability perspective (to fix bugs that could cause data loss before),
particularly on ext4, so it's possible the regression you're seeing is
just the expense of handling things properly.

If you already have sysbench on there, I'd suggest comparing the two
systems by seeing how fast each can execute fsync requests:

sysbench --test=fileio --file-fsync-freq=1 --file-num=1
--file-total-size=16384 --file-test-mode=rndwr run | grep "Requests/sec"

To help distinguish whether this regression might be coming from the
already known changes in that area, or if it's instead from something
that's impacting CPU efficiency.

Also, it's easy to see a performance change of this size just from the
database files being on a different part of the disk if you didn't
control for that. Disks are almost twice as fast at their beginning
than their end nowadays.

--
Greg Smith 2ndQuadrant US Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.us