[petsc-users] Question on DMPlexCreateFromCellList and DMPlexCreateFromFile

[Matthew Knepley]

On Thu, Feb 15, 2018 at 7:40 PM, Danyang Su <danyang.su at gmail.com> wrote:

> Hi Matt,
>
> I have a question on DMPlexCreateFromCellList and DMPlexCreateFromFile.
> When use DMPlexCreateFromFile with Gmsh file input, it works fine and each
> processor gets its own part. However, when use DMPlexCreateFromCellList,
> all the processors have the same global mesh. To my understand, I should
> put the global mesh as input, right?


No. Each process should get part of the mesh in CreateFromCellList(), but
the most common thing to do is to
feed the whole mesh in on proc 0, and nothing in on the other procs.

  Thanks,

    Matt


> Otherwise, I should use DMPlexCreateFromCellListParallel instead if the
> input is local mesh.
>
> Below is the test code I use, results from method 1 is wrong and that from
> method 2 is correct. Would you please help to check if I did anything wrong
> with DMPlexCreateFromCellList input?
>
> !test with 4 processor, global num_cells = 8268, global num_nodes = 4250
>
> !correct results
>
>  check rank            2  istart         2034  iend         3116
>  check rank            3  istart         2148  iend         3293
>  check rank            1  istart         2044  iend         3133
>  check rank            0  istart         2042  iend         3131
>
> !wrong results
>
>   check rank            0  istart         8268  iend        12518
>   check rank            1  istart         8268  iend        12518
>   check rank            2  istart         8268  iend        12518
>   check rank            3  istart         8268  iend        12518
>
>
>       !c *************    test part    *********************
>       !c method 1: create DMPlex from cell list, same duplicated global
> meshes over all processors
>       !c the input parameters num_cells, num_nodes, dmplex_cells,
> dmplex_verts are all global parameters (global mesh data)
>       call DMPlexCreateFromCellList(Petsc_Comm_World,ndim,num_cells,   &
> num_nodes,num_nodes_per_cell,      &
> Petsc_True,dmplex_cells,ndim,      &
> dmplex_verts,dmda_flow%da,ierr)
>       CHKERRQ(ierr)
>
>
>       !c method 2: create DMPlex from Gmsh file, for test purpose, this
> works fine, each processor gets its own part
>       call DMPlexCreateFromFile(Petsc_Comm_World,                      &
> prefix(:l_prfx)//'.msh',0,             &
>                                 dmda_flow%da,ierr)
>       CHKERRQ(ierr)
>
>       !c *************end of test part*********************
>
>
>       distributedMesh = PETSC_NULL_OBJECT
>
>       !c distribute mesh over processes
>       call DMPlexDistribute(dmda_flow%da,0,PETSC_NULL_OBJECT,          &
>                             distributedMesh,ierr)
>       CHKERRQ(ierr)
>
>       !c destroy original global mesh after distribution
>       if (distributedMesh /= PETSC_NULL_OBJECT) then
>         call DMDestroy(dmda_flow%da,ierr)
>         CHKERRQ(ierr)
>         !c set the global mesh as distributed mesh
>         dmda_flow%da = distributedMesh
>       end if
>
>       !c get coordinates
>       call DMGetCoordinatesLocal(dmda_flow%da,gc,ierr)
>       CHKERRQ(ierr)
>
>       call DMGetCoordinateDM(dmda_flow%da,cda,ierr)
>       CHKERRQ(ierr)
>
>       call DMGetDefaultSection(cda,cs,ierr)
>       CHKERRQ(ierr)
>
>       call PetscSectionGetChart(cs,istart,iend,ierr)
>       CHKERRQ(ierr)
>
> #ifdef DEBUG
>         if(info_debug > 0) then
>           write(*,*) "check rank ",rank," istart ",istart," iend ",iend
>         end if
> #endif
>
>
> Thanks and regards,
>
> Danyang
>
>


-- 
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.caam.rice.edu/~mk51/>
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