performance degradation in 6.2 when adding a second quad core chip

Kris Kennaway kris at
Thu Feb 21 22:50:01 UTC 2008

benjamin thielsen wrote:
> hi folks-
> we've been experiencing some interesting behavior on single quad core 
> computers as compared to dual quad core computers.

Yes, this can happen when you run into concurrency bottlenecks in the 
application or in the kernel.

> it appears that adding a second processor to the system (leaving it 
> otherwise untouched) actually decreases performance.  we've got a small 
> rudimentary test process, built in house, that does postgresql queries 
> (selects) via http requests (apache2/php5).

7.0 will perform much better than 6.x on SMP workloads in general, 
however TCP I/O is not yet at the point where it can make efficient use 
of many processors (there has been a lot of work on TCP in 7.0, but it 
is not yet at the stage where a performance payoff will be seen with 
more than about 4 CPUs).  This is one of the projects that we will be 
working on this year, so you can expect future releases to have improved 
concurrent TCP performance.

There may be other issues, so if you like you can enable LOCK_PROFILING 
and obtain a trace when your workload is running (see the manpage).  You 
should also try the ULE scheduler on 7.0.


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