[Omp] number of processors vs number of threads

[Tian, Xinmin]

I assume you meant using OMP_NUM_THREADS=2, 4, 8. If you use OMP_NUM_THREADS=4 or 8, the better performance may due to a better load balance. Or, it could due to HT. You need to provide more details. 

What is your system configuration? Is Xeon processor with HT enabled?
What compiler you are using?

Xinmin Tian (for OpenMP ARB) 
Intel Corporation

-----Original Message-----
From: Omp-bounces at openmp.org [mailto:Omp-bounces at openmp.org] On Behalf Of mbernabeu at dsic.upv.es
Sent: Tuesday, March 22, 2005 9:39 AM
To: Omp at openmp.org
Subject: [Omp] number of processors vs number of threads

Hello everyone,

I've been executing some OpenMP code in a Xeon biprocessor. There is something
about execution times that i can't understand. Normally i use "export
OMP_THREAD_NUM=2" to ensure 1 thread it's executing on each processors.
Depending on the program and the amount of data, I'm getting better times using
OMP_THREAD_NUM=4 or OMP_THREAD_NUM=8 despite there are just 2 processors.

I supose that this is relationated whith idle times and so on. Could anyone give
me a more technical answer?

Thanks a lot.

--

M. Óscar Bernabeu i Llinares
Departament de Sistemes Informàtics i Computació.
Universitat Politècnica de València.
València. Spain.
mbernabeu at dsic.upv.es



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