[petsc-users] Output cell data related to DMDA

[Barry Smith]

> On Dec 18, 2020, at 2:29 AM, Denis Davydov <davydden at gmail.com> wrote:
> 
> Hi Matt,
> 
> By global vector you mean one created with
> 
> VecCreateMPI(..., nel, PETSC_DETERMINE,...)
> 
> ? If so, that gives segfault (even with 1 MPI process) in user write function, which is just 
> 
> VecView((Vec)obj,viewer);
> 
> which clearly indicates that I misunderstand your comment.
> 
> Would you please clarify what PETSc expect as a “global” vector in case of cell-based quantities as opposed to unknowns/fields associated with the DMDA discretization?
> 

 Denis,

 Not sure what you mean by cell-based but if your vector is associated with a DMDA you need to create DMCreateGlobalVector() to get the properly layout with respect to the DM. If you use VecCreateMPI() it just has a naive 1d layout not associated with the DMDA in any way so won't be compatible. (Of course we would hope the code would not "crash" with an incompatible vector but just produce a useful error message)

  Barry




> Sincerely,
> Denis
> 
>> Am 17.12.2020 um 18:58 schrieb Matthew Knepley <knepley at gmail.com>:
>> 
>> 
>> On Thu, Dec 17, 2020 at 12:18 PM Denis Davydov <davydden at gmail.com <mailto:davydden at gmail.com>> wrote:
>> Dear all,
>> 
>> I would like to output cell data (eg conductivity coefficient) in VTK for DMDA setup.
>> 
>> Given that I know how many elements/cells are owned locally, I hoped that PetscViewerVTKAddField with PETSC_VTK_CELL_FIELD would do the job.
>> However I am not sure whether provided vector should be fully distributed (no ghosts)? If not, can I get the required ghosts from DMDA created with DMDACreate3D ?
>> 
>> I believe that it outputs global vectors, meaning that there are no ghosts.
>> 
>>   Thanks,
>> 
>>      Matt
>>  
>> Ps. I saw just one relevant discussion on the mailing list.
>> 
>> Sincerely,
>> Denis
>> 
>> 
>> -- 
>> 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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