[mpich-discuss] Why is my quad core slower than cluster

[Gus Correa]

Hello  Zach and list

 From all that I've observed on dual-processor dual-core PCs,
and from all that I've read on the web about dual-processor quad-core 
machines,
your results are not alarming, but typical.
I was as disappointed as you are, when I saw my speedup results.
A lot of people out there had the same frustration too.

My benchmarks using a standard climate atmospheric model (NCAR CAM3) on
a dual-processor dual-core Xeon workstation showed a speedup factor of 3 
(not 4),
when I moved from one core to four cores.
Likewise for a dual-processor dual-core Opteron workstation,
I've got a speedup factor slightly below 3.5. 
(Better than Xeon, but still not 4).

The problem seems to get worse with quad-cores, again with the Opterons 
slightly ahead of the game.
Memory/bus contention has been mentioned as the culprit by a lot of people.
One core in a multicore doesn't scale as one (single-core) CPU.

You will find plenty of references to this problem on the web and on 
many mailing lists:
here in the MPICH list, on the Rocks Cluster list, on the MITgcm list, 
etc, etc.

I hope it heals (as helping it cannot)
Gus Correa

-- 
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Gustavo J. Ponce Correa, PhD - Email: gus at ldeo.columbia.edu
Lamont-Doherty Earth Observatory - Columbia University
P.O. Box 1000 [61 Route 9W] - Palisades, NY, 10964-8000 - USA
Oceanography Bldg., Rm. 103-D, ph. (845) 365-8911, fax (845) 365-8736
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zach wrote:

>I am using a cluster.
>Each pc has two cpus and they are Xeon. Each cpu has 4GB, i think red
>hat is running.
>
>I also use a pc at home- quad core intel chip, 8gb ram, ubuntu.
>
>Both are using mpich.
>
>I have found that my home pc is only running about 1/3 the speed of
>the cluster, and the number of processes (4) and code is the same.
>
>Can anyone tell me if this is typical, and why, or am I not optimizing
>something properly?
>
>Thanks
>Zach
>  
>