[petsc-users] Convergence/accuracy degradation with increasing number of procs

[Matthew Knepley]

On Mon, Jan 9, 2012 at 10:31 AM, Alexander Grayver
<agrayver at gfz-potsdam.de>wrote:

> I have tested default GMRES+ILU(0) solver for problem of size ~200000 and
> I observed that both convergence rate and accuracy degrade when I increase
> number of processes.
> E.g., number of iterations almost doubles when going from 1 to 4
> processors.
>
> I noticed that ILU(0) is somehow represented by sequential matrices
> internally and feel that this might be a reason?
>

The default is Block Jacobi+ILU(0) and GMRES(30) which will definitely
degrade as the number of
processes is increased. You would need to use an optimal preconditioner
like Multigrid if you want
a constant number of iterates.

   Matt


> Regards,
> Alexander
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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