- If the AMGBackend should be used without SuperLU as coarse grid solver, it can
  be benefitial to decrease the corresponding tolerance. To do so, apply the
  patch istl-2.3.0.patch in your directory containing
  DUNE-ISTL 2.3.0, e.g.:
  patch -p1 <../dumux/patches/istl-2.3.0.patch