[petsc-users] DMDA ghosted boundary

Matthew Knepley knepley at gmail.com
Fri Aug 12 05:26:55 CDT 2022

On Fri, Aug 12, 2022 at 2:21 AM Quynh Nguyen <nguyenquynh9537 at gmail.com>

> 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

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 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.
> -----------------------------------------------------------------------------------------
> Mobile: (+82) 010-9394-5595
> Email: nguyenquynh9537 at gmail.com; nguyenthiquynhb_t58 at hus.edu.vn

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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