[Beowulf] computing on Altix?

Kevin Ying kying at pppl.gov
Fri Sep 9 07:11:15 PDT 2005

Last week, we had a user running an mpi job which does not use threads/openMP
in his code.  However, the libraries he linked to spawn threads automatically
based on the number of processors on our Altix.  The result is that too many
threads share the CPUs and the OS was busy doing context switch. 

One thing you might want to check and make sure is that the number of threads
is limited to the number of CPUs you have.  Context switch is not cheap.

- Kevin Ying

Princeton Plasma Physics Laboratory
P.O. Box 451, MS-09
Princeton, NJ 08543-0451

Email : kying at pppl.gov
Phone : (609) 243-2718

>-----Original Message-----
>From: beowulf-bounces at beowulf.org [mailto:beowulf-bounces at beowulf.org] On
>Behalf Of Hongmei He
>Sent: Friday, September 09, 2005 3:13 AM
>To: beowulf at beowulf.org
>Subject: [Beowulf] computing on Altix?
>I submit a task to Altix 350 share memory parallel machine.
>The task is to compute a graph, and rquests 6, 9, 12, 16 processors
>respectively. The results show that the running time is decreasing with the
>rise of the size of processors for same graph, while each processor runs
>same program, and communicates with fixed number of neighbours
>periodically. why?
>any idea? thanks!
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