Solver problem

[Michel Cancelliere]

Do you mean steps (iterations) 0 and 1 for SNES or KSP? If the iterations
are for SNES probably you have problems with you nonlinear solver for which
Octave can find a solution to the linear system but the actual problem is
not in there. Do you use -snes_converged_reason? Are you sure that Matrix
and right hand side routines are working well?
Michel


On Mon, Jul 27, 2009 at 4:30 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> 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
>>
>>
>
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