Re: Perf regression in 2.6.32 (Ubuntu 10.04 LTS)

On 28/09/10 04:28, Robert Haas wrote:
> On Mon, Sep 13, 2010 at 12:05 PM, Greg Smith<greg(at)2ndquadrant(dot)com> 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.
>>
> Greg, have you run into any other evidence suggesting a problem with 2.6.32?
>
>

Not Greg (sorry), but this might be worth a look:

http://www.spinics.net/lists/linux-ext4/msg20299.html

regards

Mark