> On Jun 1, 2015, at 4:06 PM, Eduardo <[email protected]> wrote:
>
> I am solving a linear system for which the preconditioned residual decreases,
> but the true residual increases (or have an erratic behavior). According to
> Petsc FAQ, this is due to a preconditioner that is singular or close to
> singular. What can I do in this case? I used GMRES with ILU preconditioner.
^^^^^^^^^^^^^^^
>
> Incidentally, I tried to solve a smaller system with a direct solver
> (superlu_dist) and it ran, so the system apparently is not singular.
ILU can produce very badly conditioned (one could say singular
PRECONDITIONER) from not singular sparse matrices. So it doesn't have anything
to do with the system itself being singular.
What type of problem are you solving? Different problems need different
preconditioners.
Barry
>
> Thanks in advance,
> Eduardo
