firedrakeproject/firedrake — pull request #84 by wence- (2013-11-21 13:41:14+00:00)
Title: Default to Jacobi preconditioner for mixed problems

PETSc's MatNest objects don't support ilu preconditioning. In fact,
apart from physics-based block preconditioning, the only allowed
preconditioner is Jacobi. If we encounter a mixed system where the user
has not supplied a preconditioner, default to Jacobi so that they don't
see a horrible PETSc error.