# Solver problem

Matthew Knepley knepley at gmail.com
Mon Jul 27 07:47:30 CDT 2009

```On Mon, Jul 27, 2009 at 4:35 AM, Tim Kroeger <
tim.kroeger at cevis.uni-bremen.de> wrote:

> Dear all,
>
> In my application, there is a linear system to be solved in every time
> step.  Steps 0 and 1 work well, but in step 2 PETSc fails to converge. I
> suspected that the system might be unsolvable in that step and checked that
> by writing matrix and the right hand side to files and loading them into
> "octave".  Surprisingly, "octave" does find a solution to the system without
> any problems.
>
> The problem occurs even on a single core.  I am using PETSc version
> 2.3.3-p11 with the GMRES solver and ILU preconditioner.
>
> Can anybody give me a hint which settings would PETSc reliably enable
> solving systems of the type that I face?

If we could, we would already have retired. There are simply no iterative
solvers that work for all systems. The
best preconditioners are usually tailored to the particular equations being
solved. I would suggest a search of
the literature for PCs for your equations.

Thanks,

Matt

>
> I have put matrix and right hand side on my homepage; they can be
> downloaded from www.mevis.de/~tim/m-and-v.tar.gz<http://www.mevis.de/%7Etim/m-and-v.tar.gz>(7MB).  In octave, I used the following commands to find and check the
> solution:
>
> octave:1> matrix2
> octave:2> vector2
> octave:3> x=Mat_0\Vec_1;
> octave:4> res=Mat_0*x-Vec_1;
> octave:5> norm(res)
> ans =  1.0032e-12
> octave:6> norm(Vec_1)
> ans =  27.976
> octave:7> norm(Mat_0,"fro")
> ans =  2.5917e+22
> octave:8> norm(x)
> ans =  3855.3
>
>
> Best Regards,
>
> Tim
>
> --
> Dr. Tim Kroeger
> tim.kroeger at mevis.fraunhofer.de            Phone +49-421-218-7710
> tim.kroeger at cevis.uni-bremen.de            Fax   +49-421-218-4236
>
> Fraunhofer MEVIS, Institute for Medical Image Computing
> Universitaetsallee 29, 28359 Bremen, Germany
>
>

--
What most experimenters take for granted before they begin their experiments
is infinitely more interesting than any results to which their experiments
-- Norbert Wiener
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