Solver problem

[Barry Smith]

On Jul 27, 2009, at 4:35 AM, Tim Kroeger 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.

    Octave is using a direct solver. Did you try PETSc's direct solver  
using -pc_type lu?

    Barry

>
> 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?
>
> 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 (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
>