Matrix convergence.

[Barry Smith]

   Matt,

    Learn how to read!

   Barry


On Fri, 3 Mar 2006, Matthew Knepley wrote:

> billy at dem.uminho.pt writes:
>
>     I don't understand. LAPACK does not have these iterative methods?
>
>       Matt
>
>> 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.
>>>>
>>>>
>>>
>>>
>>
>>
>>
>>
>
>