[petsc-users] problems using MUMPS as a linear solver

[Jed Brown]

On Fri, Nov 25, 2011 at 15:11, Dominik Szczerba <dominik at itis.ethz.ch>wrote:

> Yes, the system is badly conditioned. But am I right with my
> (experimental) finding that the ultimate way to go is -ksp_type
> preonly? Will it do what I want: just solve my system exactly once
> using MUMPS and not Krylov?
>

Direct solvers are not immune to numerical stability issues. -ksp_type
preonly does not check whether the system has been solved, it just uses
whatever the preconditioner (direct solve in this case) returned. Putting
it inside a Krylov method generally makes it more robust. If the residual
is not actually small, you should try different MUMPS flags (consult their
user's manual) or try a different solver.
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