[petsc-users] Laplace Equation preconditioner

[Alfredo J Duarte Gomez]

Hello,

I am using KSPGMRES.

I was surprised to see DIVERGED_BREAKDOWN as well, which is why I thought
there could be some grid issues.

However, other preconditioners were able to retrieve a satisfactory
solution (PCGAMG, PCGASM, PCLU MUMPS).

Is there maybe a problem size for which boomerAMG has decreased
performance? I am dealing with problem sizes of 2 million to 20 million
points for reference.

I tried out PCGAMG and it seems to be working much better than boomerAMG,
so thank you so much for your suggestion.

Sincerely,

-Alfredo

On Wed, Oct 12, 2022 at 1:32 PM Barry Smith <bsmith at petsc.dev> wrote:

>
>    What KSP are you using? DIVERGED_BREAKDOWN is very rare for KSPGMRES.
> If you are using one of its lesser cousins like bcgs you might consider
> switching to bcgsl or gmres.
>
>    I assume because of boundary conditions or the discretization you do
> not have symmetric positive definite and thus cannot use CG.
>
>   Barry
>
>
> On Oct 12, 2022, at 2:00 PM, Matthew Knepley <knepley at gmail.com> wrote:
>
> On Wed, Oct 12, 2022 at 1:38 PM Alfredo J Duarte Gomez <
> aduarteg at utexas.edu> wrote:
>
>> Good morning PETSC users,
>>
>> I have a current solver that requires the solution of a Laplace equation
>> to be reused for all future time steps.
>>
>> The configuration is axisymmetric with Dirichlet BCs at the top and
>> bottom boundaries, and Zero Neumman conditions at the axis and far field.
>> The grid is curvilinear and structured.
>>
>> So far I have been using PCHYPRE boomeramg as the preconditioner, which
>> often works well, but I have also experienced DIVERGED_BREAKDOWN on many
>> occasions. When I use direct solver PCMUMPS it always produces a
>> satisfactory answer, which gives me confidence that the solution exists for
>> the given grid in all these configurations where boomeramg fails.
>>
>
> I do not know why Hypre is breaking down. Did you try ML or GAMG? They are
> easier to diagnose I think.
>
>   Thanks,
>
>      Matt
>
>
>> I am looking for recommendations on other preconditioners to try in this
>> problem that can produce a solution faster than PCMUMPS, or recommendations
>> on which parameters to adjust on boomeramg.
>>
>> Thank you,
>>
>>
>> --
>> Alfredo Duarte
>> Graduate Research Assistant
>> The University of Texas at Austin
>>
>
>
> --
> 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
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
>
>
>

-- 
Alfredo Duarte
Graduate Research Assistant
The University of Texas at Austin
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