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
