[petsc-users] ILUTP

[Torquil Macdonald Sørensen]

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.

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
>
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