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

[De Groof, Vincent Frans Maria]

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 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

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