# [parallel] Convergence problems for parallel linear solvers

**Bug report**

The linear solver (e.g. BiCGSTAB with SSOR) has problems when cell-centered methods are applied for non-linear systems on complex domains.

**What happened / Problem description**

The linear solver has problems when solving the shallow water equations on grids with a complex geometry (non-rectangular domains). The problem often only converges if one iteration for the preconditioner is applied.

**What you expected to happen**:

The linear solver should also converge if no/more than one iteration is applied.

**How to reproduce it (as minimally and precisely as possible)**:

I will create an example for the shallow water equations to discuss the problem.

**Anything else we need to know?**:

I couldn't locate the problem, maybe it would be also good idea test the linear solver for a simple linear equation.

**Environment**:

- Dune version: master and dune-istl feature/parallel-solver-factory
- DuMux version: feature/richards-istl-factory
- Others: