[petsc-users] Number of unknowns/proc for scaling tests

[Lukas van de Wiel]

Hi Casalegno,

The answer to your question is in the PETSc FAQ:

http://www.mcs.anl.gov/petsc/documentation/faq.html

Have a great day
Lukas


On Mon, Feb 27, 2017 at 5:29 PM, Casalegno Francesco <
francesco.casalegno at epfl.ch> wrote:

> Hello,
>
>
> I am pretty new to PETSc and I would like to get some advice on choosing
> the right ratio of unknowns/proc.
>
>
> I have implemented a finite element solver for 3D problems with
> potentially up to 5B vertices, using CG as iterative solver. To run weak
> scaling tests I have to keep constant the ratio unknowns/proc, but if that
> if this ratio is too small I will not see any scaling as the communication
> overhead would dominate. Similarly, for strong scaling tests I expect to
> observe nearly-linear scaling only up to a certain number of procs, after
> which communications will dominate.
>
>
> My question is: what is a reasonable minimum number of unknowns per proc
> to get good scaling efficiency? I know that for <50k unknowns a direct
> solver would be preferable, but I do not know if this number is also a
> reasonable guess for the ratio unknown/proc.
>
>
> Thanks in advance for your help!
>
>
> Francesco
>
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