[Beowulf] Linpack and peak performance

[Guillaume Michal]

Hi all,
We set up our first cluster in our faculty this week. As we are new to  
cluster computing, there is a lot to learn. We performed some linpack test  
using the OpenMPI benchmark available in the Rocks 4.3 distribution. The  
system is as follow:
  - GigB ethernet with switch HP Procurve 2800 series
  - 1 Master node: 500GB sata HDD, two intel quad core E5410 at 2.33GHz, 2GB  
mem
  - 4 nodes each having: 80GB sata HDD, two intel quad core E5410 at 2.33GHz,  
8GB mem

First I'm a bit confused by the parameters P and Q in HPL.dat and how to  
use them properly. I noticed a 4P 2Q test is not equivalent to a 2P 4Q,  
generally speaking it does not commute. Why? What is clearly P and Q then:  
P for number of processors per nodes and Q for the number of nodes?

Secondly, what is the definition of processor for a quad core  
architecture? I suppose a quad core should be counted as 4 processors.

I launched Linpack using Ns=10000 and various configuration for P and Q.  
At the moment I got a maximum of 78 Gflops using P=8 Q=4 -> 32 processors.

If I'm right the peak performance should be Rpeak= 4 cores x 4 floting  
point op per cycle x 2.33 Ghz x 8 quad cores = 298 Gflops.
Which would lead to a test running at ~25% Rpeak.

This is very low and I see 3 causes for the problem:
	- I miscalculated Rpeak
	- P and Q are not set properly
	- there is a serious bottelneck

Thanks for your advices

Guillaume


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