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: > > This email was sent to you by someone outside the University. > You should only click on links or attachments if you are certain that the > email is genuine and the content is safe. > 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? 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 >> > > > -- > 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/> > > > The University of Edinburgh is a charitable body, registered in Scotland, > with registration number SC005336. Is e buidheann carthannais a th’ ann an > Oilthigh Dhùn Èideann, clàraichte an Alba, àireamh clàraidh SC005336. > -- 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/>