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

[Frederik Treue]

On Tue, 2013-05-07 at 09:23 -0500, Matthew Knepley wrote:
> 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.

But how can this be? If it works without a preconditioner (which it
does), doesn't that prove that the operators are correctly implemented?
Or am I missing something something here?

/Frederik Treue