[petsc-users] DMDA ghosted boundary

[Barry Smith]

> On Aug 12, 2022, at 9:27 AM, David Scott <d.scott at 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 <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?

 It is up to you to determine what values to put into those locations and put them in before you apply your "stencil" computation to evaluate the residual. You just stick values in those locations, likely after doing a DMDAVecGetArray().


> 
> 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: nguyenquynh9537 at gmail.com <mailto:nguyenquynh9537 at gmail.com>; nguyenthiquynhb_t58 at hus.edu.vn <mailto: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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