[petsc-users] DMDA ghosted boundary

[David Scott]

On 12/08/2022 11:26, Matthew Knepley wrote:
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On Fri, Aug 12, 2022 at 2:21 AM Quynh Nguyen <nguyenquynh9537 at gmail.com<mailto:nguyenquynh9537 at gmail.com>> wrote:
Dear petsc developers and users,

I am a new petsc user ^_^.
I have some questions about using DMDA_BOUNDARY_GHOSTED.

1. I have 2D Poisson code with domain (nx, ny), and I used  DMDA_BOUNDARY_NONE, then the size of the matrix is (nx*ny)X(nx*ny). what is the size of the matrix if I used  DMDA_BOUNDARY_GHOSTED? is it the same size as DMDA_BOUNDARY_NONE?

Yes, the matrix should be the same size, but local vectors have extra ghost node slots.

2. if the matrix size of DMDA_BOUNDARY_GHOSTED isn't the same as DMDA_BOUNDARY_NONE, how i can view the matrix including ghost points? I ask this one because I used MatView(P, PETSC_VIEWER_STDOUT_WORLD) to show the matrix but excluded ghost points.

Ghost nodes are there so that stencil operations on local vectors work at boundaries. You fill in the slots with boundary information.

I can see how this works if you are writing your own solver but can it be used with a KSP? In the case of periodic boundary conditions the boundary values are "filled in" for you by PETSc. Is there any way for a user to supply boundary information to a PETSc solver when using DMDA_BOUNDARY_GHOSTED?

Thanks,

David

  Thanks,

     Matt

I hope someone can explain my wonder :)

P/S: maybe, those question is simple and a bit stupid ^_^.

Have a nice day,

--
Quynh Nguyen
Graduate student, Ocean Engineering.
School of Naval Architecture and Ocean Engineering, University of Ulsan.
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