[Omp] number of processors vs number of threads

[Sarah Zhang]

If the Xeon processors are hyper-threading enabled, it may get better
performance with 4 threads. I dont know the reason for the 8 threads case.
But i guess 4/8 threads may have similar performance? And I guess the 8
threads is always not as good as the 4 threads program?

Sarah

On Tue, 22 Mar 2005 mbernabeu at dsic.upv.es wrote:

> 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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