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-   -   GAMG Number of iterations depends strongly on number of parallel processors (http://www.cfd-online.com/Forums/openfoam-programming-development/99430-gamg-number-iterations-depends-strongly-number-parallel-processors.html)

clean-energy April 3, 2012 10:54

GAMG Number of iterations depends strongly on number of parallel processors
 
Hello,
At the moment I'm performing some parallel efficiency tests on a sandybridge cluster.
i noticed that the number of needed GAMG iteration depends heavily on the number of parallel processors. This shouldn't be the case (as far as i know).

Do you have any idea?
I'm using openFoam 2.1.x with ICC.

thanks in advance

Chris Lucas April 12, 2012 03:11

Hi,

i'm not 100% sure, but the effect should be related to preconditioning. In a parallel run, the preconditioning is performed for each domain independently.

Best Regards,
Christian

arjun April 12, 2012 03:23

Quote:

Originally Posted by clean-energy (Post 352941)
Hello,
At the moment I'm performing some parallel efficiency tests on a sandybridge cluster.
i noticed that the number of needed GAMG iteration depends heavily on the number of parallel processors. This shouldn't be the case (as far as i know).

Do you have any idea?
I'm using openFoam 2.1.x with ICC.

thanks in advance


Efficiency of multigrid goes down as number of partitions increase. It is known fact. (Not all smoothers do this though, some smoothers could really create multigrid independent of processors).

In case of GAMG, I do not know whether the fall in efficiency you observed is more than it should. But efficiency of multigrid does fall down.


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