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

[Jed Brown]

Frederik Treue <frtr at fysik.dtu.dk> writes:

> 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.

How "small"?  How are you measuring the error?  Does it converge to
match the internal error as you shrink -ksp_rtol?  It should not happen
with -pc_type jacobi (or pbjacobi) because they don't do anything
special at boundaries.