Matrix convergence.

[Barry Smith]

   Run the parallel PETSc job with -ksp_type gmres -ksp_gmres_restart 200
-sub_pc_type lu


On Fri, 3 Mar 2006, billy at dem.uminho.pt wrote:

> Hi,
>
> I tried to solve the 18x18 matrix with laspack and the results are the following:
>
> Solver: GMRES, Preconditioning: NULL, Iterations: 500, Residual: 6.16E-2
> Solver: GMRES, Preconditioning: SSOR, Iterations: 500, Residual: 6.57E-2
> Solver: GMRES, Preconditioning: ILU, ERROR
>
> Solver: BiCGSTAB, Preconditioning: NULL, Iterations: 101, Residual: 8.88E-7
> Solver: BiCGSTAB, Preconditioning: SSOR, Iterations: 36, Residual: 9.87E-7
> Solver: BiCGSTAB, Preconditioning: ILU, ERROR
>
>
> Billy.
>
>
> Quoting Barry Smith <bsmith at mcs.anl.gov>:
>
>>
>>    Billy,
>>
>>     Most likely there is something wrong with the matrix being
>> generated. I would first look at the matrix entries and make
>> sure they make sense. Then I would solve the linear systems
>> with a direct solver -pc_type lu (on one processor and make
>> sure the solutions are reasonable and your simulation has
>> "correct" answers).
>>
>>    It is possible the parallel matrix generation is wrong,
>>
>>    Barry
>>
>>
>> On Thu, 2 Mar 2006, billy at dem.uminho.pt wrote:
>>
>>>
>>> Hi,
>>>
>>>
>>> I have been testing my parallel implementation with a 2D case. I have used
>> the
>>> following grids with 2 and 4 processes: 4x4, 10x10, 16x16 and 18x18.
>>>
>>> It works for the first three (max. number of iterations to converge aprox.
>> 70)
>>> but when I increase to 18x18 the matrix does not converge. I have increased
>> the
>>> maximum number of iterations and I have used GMRES, BiCGSTAB with
>>> preconditioners ASM, JACOBI, etc.
>>>
>>> With KSPType = GMRES and PCType = ASM:
>>> Number of iterations: 500 Residual: +1.420471E-01
>>>
>>> With KSPType = GMRES and PCType = JACOBI:
>>> Number of iterations: 500 Residual: +2.351882E-02
>>>
>>> With KSPType = BCGS and PCType = ASM:
>>> Number of iterations: 136 Residual: +3.911446E+04
>>>
>>> With KSPType = BCGS and PCType = NULL:
>>> Number of iterations: 157 Residual: +3.254728E+03
>>>
>>> Is it normal that the convergence detiorates so much with such a slight
>> increase
>>> in the dimension of the matrix? Is there any optimization parameter that I
>> may
>>> be missing?
>>>
>>>
>>> Billy.
>>>
>>>
>>
>>
>
>
>