[petsc-users] Convergence problem in nonlinear problem, with PCBDDC.

[Matthew Knepley]

On Mon, Oct 13, 2014 at 10:18 AM, De Groof, Vincent Frans Maria <
Vincent.De-Groof at uibk.ac.at> wrote:

>  Hi,
>
>
>  I am having a convergence problem in a nonlinear FE problem (we only use
> ksp from petsc, not snes). We don't have any problems using a direct solver
> with a cholesky factorization (Petsc factor, cholmod, mumps). Now we want
> to solve it using the bddc preconditioner. No problems on some problems. So
> far so good.
>
>
>  Unfortunately, in some problems, the first increment in the newton
> iteration does not converge. It exits due to divergence (-4) after the
> first solve (0 iterations). This happens with CG, using the natural or
> preconditioned norm. Using the unpreconditioned norm, or minres, the
> problem converges with no problems. Checking the Dirichlet and Neumann
> errors of the subdomains (using -pc_bddc_check_level 1)  delivers small
> errors for all subdomains (~1e-16 - 1e-11).
>
>
>  Because minres works, and cg doesn't, it seems like I am missing a null
> space. But since the subdomain solvers work without problems and do not
> complain
>

MINRES works for symmetric indefinite problems, whereas CG needs SPD
problems. Could your problem be indefinite? If not, and it is close,
its possible that the preconditioned problem is indefinite.

  Matt


> about a null pivot this seems unlikely. Also when I check the number of
> constraints on the subdomains, they should be fully-determined...
>
>
>
>  any suggestions on what might be going wrong here? Am I really missing a
> nullspace? Or might there be something else going wrong?
>
>
>
>  kind regards,
> Vincent
>
>


-- 
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
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