Re: Two identical systems, radically different performance

On Oct 9, 2012, at 3:24 AM, Tomas Vondra  wrote:

> On 9.10.2012 00:33, Evgeny Shishkin wrote:
>>> 
>>> pgbench: Old server
>>> 
>>>    pgbench -i -s 100 -U test
>>>    pgbench -U test -c ... -t ...
>>> 
>>>    -c  -t      TPS
>>>     5  20000  3777
>>>    10  10000  2622
>>>    20  5000   3759
>>>    30  3333   5712
>>>    40  2500   5953
>>>    50  2000   6141
>>> 
>>> New server
>>>    -c  -t      TPS
>>>    5   20000  2733
>>>    10  10000  2783
>>>    20  5000   3241
>>>    30  3333   2987
>>>    40  2500   2739
>>>    50  2000   2119
>> 
>> On new server postgresql do not scale at all. Looks like contention. 
> 
> Why? The evidence we've seen so far IMHO suggests a poorly performing
> I/O subsystem. Post a few lines of "vmstat 1" / "iostat -x -k 1"
> collected when the pgbench is running, that might tell us more.
> 

Because 50 clients can push io even with small read ahead. And hear we see nice parabola. Just guessing anyway.

> Try a few very basic I/O tests that are easy to understand rather than
> running bonnie++ which is quite complex. For example try this:
> 
> time sh -c "dd if=/dev/zero of=myfile.tmp bs=8192 count=4194304 && sync"
> 
> dd if=myfile.tmp of=/dev/null bs=8192
> 
> The former measures sequential write speed, the latter measures
> sequential read speed in a very primitive way. Watch vmstat/iostat and
> don't bother running pgbench until you get a reasonable performance on
> both systems.
> 
> 
> Tomas
> 
> 
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