I recently switched from LU solvers in FEniCS to iterative solvers (CG). The solutions looks similar to the eyeball, however, there is the difference between the solutions has a significant norm! I tried to reduce the tolerances (relative and absolute, also the maximum iterations) in the CG method but the difference does not vanish.
Anyone has an idea where I can look for the possible explanation? This problem is a hyperbolic PDE with a very fine and unstructured mesh.