[petsc-users] Question about DMPlexDistribute & distribute mesh over processes

[Matthew Knepley]

On Fri, May 27, 2022 at 9:42 AM Sami BEN ELHAJ SALAH <
sami.ben-elhaj-salah at ensma.fr> wrote:

> Hello Isaac,
>
> Thank you for your reply!
>
> Let me confirm that when I use DMCreateMatrix() with the orig_dm, I got my
> jacobian_matrix. Also, I have succeeded to solve my system and my solution
> was converged* by using one process*.
> Let me give you some other information about my code. Currently, I am
> using my own discretization system and not the PetscDS object. Considering
> the nonlinear solver SNES, I use the following routines (like the basic
> snes usage):
> - SNESSetFunction(snes,residual_vector,compute_residual ,(void*)
> _finite_element_formulation)
> - SNESSetJacobian(snes, jacobian.matrix(), _jacobian_precond_matrix,
> compute_jacobian, (void*) _finite_element_formulation)
> - SNESSolve(snes,NULL,x)
>
> Regarding  your last answer, I will try to reformulate my question as
> follows:
> Using a distributed dm instead of the original dm (not distributed dm) and
> my own discretization system (not the PetscDS object),
>

You do not have to use PetscDS, but the DM does need to a PetscSection in
order to compute sizes and sparsity patterns. Do you have that?

  Thanks,

     Matt


> should I add something specific to get a distributed jacobian_matrix over
> processes?
> I precise that I just replaced the orig_dm by a distributed mesh with the
> routine that I wrote in my first mail. So is it enough ?
>
> Thank you and have a good day,
> Sami,
>
>
> --
> Dr. Sami BEN ELHAJ SALAH
> Ingénieur de Recherche (CNRS)
> Institut Pprime - ISAE - ENSMA
> Mobile: 06.62.51.26.74
> Email: sami.ben-elhaj-salah at ensma.fr <sami.ben-elhaj-salah at ensma.fr>
> www.samibenelhajsalah.com
> <https://samiben91.github.io/samibenelhajsalah/index.html>
>
>
>
> Le 25 mai 2022 à 19:41, Toby Isaac <toby.isaac at gmail.com> a écrit :
>
> Hi Sami,
>
> Just to verify: if you call DMCreateMatrix() on orig_dm, do you get a
> Jacobian matrix?
>
> The DMPlex must be told what kind of discretized fields you want a
> matrix for and what equations you are discretizing.  This is handled
> by the PetscDS object.  In snes/tutorials/ex59.c, see the code after
> DMGetDS() for an example.
>
> - Toby
>
> On Wed, May 25, 2022 at 1:17 PM Sami BEN ELHAJ SALAH
> <sami.ben-elhaj-salah at ensma.fr> wrote:
>
>
> Dear PETSc developer team,
>
> I m trying to create à jacobian_matrix from distributed DM. I have
> followed the two examples (snes/tests/ex2.c and ex56.c). So I wrote this
> routine:
>
> PetscDM orig_dm;
> PetscDM dist_dm = NULL;
> PetscPartitioner part;
> DMPlexGetPartitioner(orig_dm, &part);
> PetscPartitionerSetType(part, PETSCPARTITIONERPARMETIS);
> DMPlexDistribute(orig_dm, 0, NULL, &dist_dm);
>
> PetscErrorCode err = DMCreateMatrix(dist_dm, &jacobian_matrix);
> PetscInt M, N, m, n;
> MatGetSize(jacobian_matrix, &M, &N);
> MatGetLocalSize(jacobian_matrix, &m, &n);
>
> Then I run my code with 2 processes and I obtained this result:
> Size from jacobian_matrix: M=0 m =0 (this result is the same in all
> processes).
>
> I didn't understand if I forgot something in my code to obtain the correct
> values for the local and global sizes for the jacobean_matrix? (I mean if
> my code is missing something to obtain a distributed mesh over processes ?)
>
> Thank you in advance for any help!
> Sami,
>
>
> --
> Dr. Sami BEN ELHAJ SALAH
> Ingénieur de Recherche (CNRS)
> Institut Pprime - ISAE - ENSMA
> Mobile: 06.62.51.26.74
> Email: sami.ben-elhaj-salah at ensma.fr <sami.ben-elhaj-salah at ensma.fr>
> www.samibenelhajsalah.com
>
>
>
>
>

-- 
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/>
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