gmres - restart and Gauss-Seidel

[Matthew Knepley]

That is strange. Your parallel SOR should be a stronger PC than the
Block-Jacobi SOR that PETSc is using. However, it is hard to prove
that restarted GMRES will converge, and thus this behavior is not
impossible.

However, if you are using MG and the smoother is doing its job, you
should not take more than a few iterates. You should never get to
30 outer GMRES iterates.

  Matt

On Thu, Aug 14, 2008 at 5:09 PM, Lukasz Kaczmarczyk
<lukasz at civil.gla.ac.uk> wrote:
> Hello,
> I have implementation of geometric multi-grid for heterogeneous
> quasi-brittle materials for hybrid-trefftz finite elements (degrees of
> freedom are on faces -> small number of neighbours). Multi-grid
> algorithm need smoothing, for that I use Gauss-Seidel, however SOR
> implemented in PETSc is not parallel. That is way, I implemented my own
> parallel Gauss-Seidel with colouring of faces in order to reduce
> communication. Everything seems to work prefect, except that that for
> GMRES after restart algorithm is divergent. With SOR implemented in
> PETSc algorithm does not have this  problem. Can any give my tip how to
> cure may problem.
>
> Regards,
> Lukasz
>
>



-- 
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