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

[Frederik Treue]

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.

/Frederik Treue