[petsc-users] Mat created by DMStag cannot access ghost points

[Matthew Knepley]

On Tue, May 31, 2022 at 10:28 AM Ye Changqing <Ye_Changqing at outlook.com>
wrote:

> Dear developers of PETSc,
>
> I encountered a problem when using the DMStag module. The program could be
> executed perfectly in serial, while errors are thrown out in parallel
> (using mpiexec). Some rows in Mat cannot be accessed in local processes
> when looping all elements in DMStag. The DM object I used only has one DOF
> in each element. Hence, I could switch to the DMDA module easily, and the
> program now is back to normal.
>

Is it possible to send a simple example? We have parallel examples with
DMStag in PETSc, just from this description I cannot see what is going
wrong for you.

  Thanks,

     Matt


> Some snippets are below.
>
> Initialise a DMStag object:
> PetscCall(DMStagCreate2d(PETSC_COMM_WORLD, DM_BOUNDARY_NONE,
> DM_BOUNDARY_NONE, M, N, PETSC_DECIDE, PETSC_DECIDE, 0, 0, 1,
> DMSTAG_STENCIL_BOX, 1, NULL, NULL, &(s_ctx->dm_P)));
> Created a Mat:
> PetscCall(DMCreateMatrix(s_ctx->dm_P, A));
> Loop:
> PetscCall(DMStagGetCorners(s_ctx->dm_V, &startx, &starty, &startz, &nx,
> &ny, &nz, &extrax, &extray, &extraz));
> for (ey = starty; ey < starty + ny; ++ey)
> for (ex = startx; ex < startx + nx; ++ex)
> {
> ...
> PetscCall(DMStagMatSetValuesStencil(s_ctx->dm_P, *A, 2, &row[0], 2,
> &col[0], &val_A[0][0], ADD_VALUES));  // The traceback shows the problem is
> in here.
> }
>
> Best,
> Changqing
>
>

-- 
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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