My system is a symmetric FE problem with lagrange multipliers (e.g. incompressible Stokes' flow):
\begin{pmatrix}A & B^T \\ B & C\end{pmatrix}
where $C = 0$ is the typical case (I have even made sure that the equations are numbered so that the Lagrange multipliers appear last ). The system is quite large (+100k lines).
Having read the answer to this question, I was given the impression that there are suitable preconditioners that can be used for mixed FE-problems.
Using PETSc, I've managed to solve the system with MINRES (-ksp_type minres -pc_type none -mat_type sbaij
), although the precision isn't great (causing several Newton-iterations for a linear problem). No other combination of preconditioner and ksp-solver seems to work.
Is there any combination of flags for PETSc that will solve this system faster than with just MINRES?