[petsc-users] ILUTP

[Matthew Knepley]

On Wed, Oct 30, 2013 at 8:58 AM, Torquil Macdonald Sørensen <
torquil at gmail.com> wrote:

> Thanks Matthew!
>
> The problem I'm working on is the Dirac equation, in various number of
> dimensions. I'm going to take a stab at multigrid preconditioning.
>

My limited knowledge of the Dirac Equation tells me that it is a
relativistic wave equation, and thus hyperbolic, which is quite difficult
for
MG (but doable with a bunch of work). How do you have this formulated?

  Thanks,

     Matt


> Best regards
> Torquil Sørensen
>
>
> On 27 October 2013 14:48, Matthew Knepley <knepley at gmail.com> wrote:
>
>> On Sat, Oct 26, 2013 at 3:05 PM, Torquil Macdonald Sørensen <
>> torquil at gmail.com> wrote:
>>
>>> Hi!
>>>
>>> I have a linear problem Ax=b, with complex values, that is solved very
>>> well using the ILUTP and GMRES implementations in GMM++. None of the other
>>> preconditioners available in GMM++ would work (e.g. ILU and ILUT) for this
>>> problem.
>>>
>>> Then I tried the same problem using ILU and GMRES in PETSc, with no
>>> success, despite a lot of attempts at adjusting ILU settings. I always end
>>> up with gigantic residual norm values. The same PETSc program works well
>>> when I apply it to a different matrix A.
>>>
>>> I'm now suspecting that the ILU options cannot be set so as to obtain
>>> ILUTP behaviour.
>>>
>>> What would be the recommended method to access an ILUTP preconditioner
>>> from PETSc?
>>>
>>> According to the PETSc website, a preconditioner named ILUDT is
>>> available by using the external package Hypre, but I had to deselect Hypre
>>> during the PETSc build due to my use of complex numbers... So do you guys
>>> think that I should transform everything to a real representation and try
>>> Hypre/ILUDT?
>>>
>>
>> I suggest you look for a different preconditioner. The fact that you have
>> so much trouble reproducing the behavior shows
>> you just how fragile the performance of ILU is. It may work for a certain
>> size, but fail for larger or smaller problems, or slightly
>> different parameters. What problem are you solving? Usually the best
>> option is to consult the literature for preconditioners
>> tailored to your problem, and then reproduce them.
>>
>>    Matt
>>
>>
>>> Best regards
>>> Torquil Sørensen
>>>
>>>
>>
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to which their
>> experiments lead.
>> -- Norbert Wiener
>>
>
>


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