On 12/08/2022 18:15, Matthew Knepley wrote:
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On Fri, Aug 12, 2022 at 9:27 AM David Scott <d.sc...@epcc.ed.ac.uk> wrote:
On 12/08/2022 11:26, Matthew Knepley wrote:
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On Fri, Aug 12, 2022 at 2:21 AM Quynh Nguyen
<nguyenquynh9...@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?
Right now, there is not. This is mainly due to the lack of consensus
for boundary condition specification. Periodic conditions are simple
in this respect.
For example, Plex has such a thing
https://petsc.org/main/docs/manualpages/DM/DMAddBoundary/
that can be used to fill in these constrained spots
https://petsc.org/main/docs/manualpages/DMPLEX/DMPlexInsertBoundaryValues/
But there have been many complaints about this interface (and I
suspect any interface we choose).
Maybe there is something that we could write that would make it easier
for you with DMDA?
The way I had imagined it working was that for each dimension the user
could register a function to be to be called at the start of every
iteration (in a similar fashion to adding a monitor). Not all of the
dimensions would have to be specified as DMDA_BOUNDARY_GHOSTED and thus
require a function to be registered. This function would have access to
the DMDA and the local vector so it could determine if ghost locations
were present and, if so, set appropriate values. Does this make sense?
Thanks,
David
Thanks,
Matt
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.
-----------------------------------------------------------------------------------------
Mobile: (+82) 010-9394-5595
Email: nguyenquynh9...@gmail.com; nguyenthiquynhb_...@hus.edu.vn
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