> Direct solvers are not immune to numerical stability issues. -ksp_type > preonly does not check whether the system has been solved, it just uses > whatever the preconditioner (direct solve in this case) returned. Putting it > inside a Krylov method generally makes it more robust.
This is very useful to know. I now can manage to converge with -ksp_type fgmres and -ksp_max_it 1. It does not seem to perform any slower than -ksp_type preonly. Thanks, Dominik
