[petsc-users] Convergence of iterative linear solver

Barry Smith bsmith at mcs.anl.gov
Mon Jun 1 21:11:07 CDT 2015

> On Jun 1, 2015, at 4:06 PM, Eduardo <erocha.ssa at gmail.com> 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.


> Thanks in advance,
> Eduardo

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