Subject: Scaling of G98 on Clusters

From: Christoph Steinbeck <[email-masked]>
Date: Thu, 30 Mar 2000 19:09:48 +0200

Greetings everybody, 

this question goes to those who use Gaussian 98 (or maybe also other
versions) on workstations clusters. 

I'm currently in the process of evaluating g98 with linda my new 16-node
linux cluster. Each node is a 550 Mhz Pentium III with 128 MB of RAM and
a 6 GB hard disk. Communication is done via a 100 MBit ethernet and the
16 nodes are separated from the institutes network by a switch. 

The calculations I'm interested in are of the type indicated by the
example listed below, which essentially are composite jobs with geometry
optimizations on B3LYP/6-31G(d) level followed by a carbon shift
calculation on the same level.

Now, I ran the jobs with a-pinene and abietic acid (a mono- and a
diterpene) with 1, 4, 8, 12 and 16 nodes to see how the setup scales.
What I see is an almost linear scaling up to 4 nodes. With eight nodes
the effective speedup is still nice with 6.41 where perfect behaviour
obviously would be 8.00. But then it gets worse as you see and when
going from 12 to 16 nodes the calculation even slows down. Here are the
numbers. 

Nodes	Effective speedup	
1	1
4	3,93
8	6,41
10	8,55
12	8,67
16	7,9	

This is with a-pinen (C10H16). 
When more than doubling the size of the molecule with abietic acid
(C20H30O2) the tendency is a little better (8 nodes take 41 h, 16 nodes
take 30 h).

Is this bad scaling to be expected? The graph on the gaussian web site
shows the speedup of the calculations up to six nodes and I begin to
understand why that is :-)
In many publications on beowulf clusters you find the statement that in
many cases one can expect near linear scaling of those cluster up to 16
nodes. I understand that  this highly depends on the type of
calculation, so am I stuck with my problem, or are there ways to
optimize the behaviour by tuning the settings?

BTW, I'm using the standard (install time) settings for g98 and linda.

Any comments are very much appreciated.

Cheers, 

Chris

--
Dr. Christoph Steinbeck (http://www.ice.mpg.de/departments/ChemInf)
MPI of Chemical Ecology, Tatzendpromenade 1a, 07745 Jena, Germany
Tel: +49(0)3641 643644 - Fax: +49(0)3641 643665

What is man but that lofty spirit - that sense of enterprise. 
... Kirk, "I, Mudd," stardate 4513.3..

----------------
%Chk=abieticacid
#T B3LYP/6-31G(d) Opt Test

  Name: Z:\gaussian\abietinsaeure.mol
  MOPAC file created on 22/12 17:49:56 1998 by HYPERCHEM

   0   1
 C,0,0.,0.,0.
 C,1,1.5115
[ ... stuff deleted here ...]
 H,20,1.1137,5,111.4364,6,74.9555,0
 H,22,0.9669,19,110.5133,21,-178.666,0

--Link1--
%Chk=abieticacid
%NoSave
#T B3LYP/6-31G(d) NMR Test Geom=AllCheck Guess=Read
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