[petsc-users] Diagnosing Poisson Solver Behavior

[K. N. Ramachandran]

Hello Matt,

Ah ok. I had tried specifying -pc_mg_levels earlier, but it asks for
interpolation to be setup. I am currently working on using DMDA from
examples 32 and 34, which sets up the interpolation, using
DMDASetInterpolationType. So this is the recommended way to go forward
right?

Since I supply the sparse matrix format and the rhs values to PETSc, does
it make sense to do a DMDACreate1d instead of 3d? I did a sample
implementation using DMDA 1d and when I call KSPSolve and fetch the
solution, it comes out all zeros, with the solver having converged. I am
trying to understand what I am doing wrong there.

On Mon, Oct 12, 2015 at 10:17 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Mon, Oct 12, 2015 at 9:11 PM, K. N. Ramachandran <knram06 at gmail.com>
> wrote:
>
>> Hello Matt,
>>
>> Actually I felt the boundary conditions were having a role to play and
>> set all the boundary conditions to Dirichlet. In this case, convergence was
>> almost immediate with the Hypre preconditioner, taking 17 seconds with 3
>> iterations. The MG method took about the same time though.
>>
>> So I reverted to the Dirichlet, Neumann mix of BCs and Hypre starts to
>> diverge. Please find attached the output for the Hypre run using Dirichlet
>> and Neumann for a 21^3 grid (rows), with a max of 7 nonzeros per row.
>> Details of the options used before running are in the file. The solver used
>> in all cases is bcgs.
>>
>> Also attached is the MG output for 101^3 grid
>> 1) Dirichlet and Neumann
>> 2) Dirichlet only
>>
>> where it seems to take about the same time.
>>
>
> Notice that you have no levels of MG here. You need to use -pc_mg_levels
> <n>
>
>    Matt
>
>
>>
>>

Thanking You,
Ramachandran K.N.
Ph: 814-441-4279
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