Re: Perf regression in 2.6.32 (Ubuntu 10.04 LTS)

On 13 September 2010 17:27, Stefan Kaltenbrunner
<stefan@kaltenbrunner.cc> wrote:
> On 09/13/2010 06:05 PM, Greg Smith wrote:
>>
>> 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.
>
> well the main point here is that domas is doing a pure read-only test on a
> rather small workload so it should entirely fit in memory...
> From some very quick testing here as well it rathers seems that for some
> reason the CPU scheduler is not actually scheduling us all the available CPU
> on 2.6.32 or we are having some sort of locking issue that is more exposed
> on this kernel.

I thought sysbench was designed for MySQL benchmarks.  How new is the
PostgreSQL driver?  Is it stable yet?

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Thom Brown
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