[petsc-users] PC introduce errors at processor limits?

[Matthew Knepley]

On Tue, May 7, 2013 at 9:20 AM, Frederik Treue <frtr at fysik.dtu.dk> wrote:

> Hi,
>
> I may be overlooking something very obvious here, but:
>
> I'm trying to solve a convection-diffusion problem. The method I use
> requires me to solve some Helmholtz and Poisson equations, which is the
> time consuming part. In order to reduce this time, I try to use
> preconditioners. However, no matter which preconditioner I use (I've
> tried PCJACOBI,PCBJACOBI,PCPBJACOBI,PCMG with 2 levels, galerkin) it
> introduces errors along the edges of the local domains when using
> multiple processors. These errors are small, but they don't converge to
> 0 as a function of resolution. I have checked with PCNONE, which
> eliminates the problem, but becomes unbearably slow.
>
> Is this somehow unavoidable? Or am I making some silly mistake? The code
> is somewhat complicated, but if desired, I can try to cook up a
> proof-of-(non)concept.


You have made a mistake somewhere, probably in your parallel function
evaluation.
For Poisson, see SNES ex5.

   Matt


>
> /Frederik Treue
>
>


-- 
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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