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Re: Two identical systems, radically different performance

From: Tomas Vondra <tv(at)fuzzy(dot)cz>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: Two identical systems, radically different performance
Date: 2012-10-08 23:24:02
Message-ID: 50736092.8080906@fuzzy.cz (view raw or flat)
Thread:
Lists: pgsql-performance
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.

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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