[Beowulf] How does one calculate the scalability of one's code?

Singh, Tajendra tvsingh at ats.ucla.edu
Wed Jun 9 19:19:42 EDT 2004


Hello Chris,

This is not surprising unless you have only neighbor communications in
the parallel code. Otherwise it may be because of the fact that when you
increase the nodes 8 times, the overall parallel overheads also
increases e.g. example the time for synchronization or collective
computations etc.

In case you do a proper profiling of the parallel run you will find out
where this extra time is being spent.

Hope this gives a hint!!

Thanks,

TV Singh
5919A Math Sciences bldg,
ATS, UCLA, Los Angeles, CA 90095-1557
Ph: (310)825-5698  

-----Original Message-----
From: beowulf-bounces at beowulf.org [mailto:beowulf-bounces at beowulf.org]
On Behalf Of Chris Oubre
Sent: Wednesday, June 09, 2004 9:24 AM
To: beowulf at scyld.com
Subject: [Beowulf] How does one calculate the scalability of one's code?

Hello,
  My name is Chris Oubre.  I am a physics graduate student at Rice
University studying Maxwell's equations using a Finite Difference Time
Domain code.  I am trying to calculate the scalability of my code two
ways

1) Fixed job size

For Fixed job size, I can use Amdahl's law S = N/ (b * N + (1-b)) and
fit
the curve to find b ( the % of code that is serial)

Or I could use the Gustafson-Baris Law S = N - b(N - 1) and likewise fit
to
find b


2) Fixed load size

For fixed load size, I want to see how the algorithm behaves if I make
the
jobs larger and increase the number of nodes to keep the job size
constant.
Since I am using a 3D code, I double the job size and increase the
number of
nodes by a factor of 8

I would expect that if the code scaled perfectly, they would execute in
the
same amount of time.  I observe that the algorithm takes 1.6 times as
long.
How do I report this?  What does this number tell me generally about the
scalability of my code?  Do I need to calculate something else?

To make the number more clear
Run1 Specs
Grid Size: 100 x 100 x 100
Nodes: 4
Time: 605 seconds

Run2 Specs
Grid Size: 200 x 200 x 200
Nodes: 32
Time: 944 seconds

So Run2 took ~ 1.6 times as long as Run1

Thank you for all of you help.

****************************************************
Christopher D. Oubre                               *
email: chris_oubre at hotmail.com                     *
research: http://cmt.rice.edu/~coubre              *
Web: http://www.angelfire.com/la2/oubre            *
Hangout: http://pub44.ezboard.com/bsouthterrebonne *
Phone:(713)348-2506  Fax:   (713)348-4150          *
Rice University                                    *
Department of Physics, M.S. 61                     *
6100 Main St.                       ^-^            *
Houston, Tx  77251-1892, USA       (O O)           *
-= Phlax=-                         ( v )           *
************************************m*m*************

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