[petsc-dev] Can I call PetscSectionAddDof(s, p, ndof) at a shred 'p' by more than one processors?

[Matthew Knepley]

On Thu, Nov 19, 2020 at 4:35 PM Zhang, Hong <hzhang at mcs.anl.gov> wrote:

> Matt,
> At a local section point (including ghost point), how to find all the
> owners? For example, proc[0] and proc[2] share p. How proc[0] find that
> proc[2] is an owner? Any routine would provide this info?
>

All ownership data is handled by the PetscSF, the Section just stores local
information about data. You can ask
the SF is a point is a leaf, meaning that it is not owned. You can also ask
for an owned point, how many other
processes have it as a leaf:


https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PetscSF/PetscSFComputeDegreeBegin.html

   Thanks,

     Matt


> Hong
>
> ------------------------------
> *From:* Matthew Knepley <knepley at gmail.com>
> *Sent:* Wednesday, November 18, 2020 1:56 PM
> *To:* Lawrence Mitchell <wence at gmx.li>
> *Cc:* Zhang, Hong <hzhang at mcs.anl.gov>; petsc-dev <petsc-dev at mcs.anl.gov>
> *Subject:* Re: [petsc-dev] Can I call PetscSectionAddDof(s, p, ndof) at a
> shred 'p' by more than one processors?
>
> On Wed, Nov 18, 2020 at 2:19 PM Lawrence Mitchell <wence at gmx.li> wrote:
>
> > On 18 Nov 2020, at 15:26, Zhang, Hong via petsc-dev <
> petsc-dev at mcs.anl.gov> wrote:
> >
> > Matt or Jed,
> > Can I call PetscSectionAddDof(s,p,ndof) at a shred 'p' by more than one
> processors? For example,
> > if (rank == 0) {
> >     PetscSectionAddDof(s,p,1) ;
> > } else if (rank == 1) {
> >     PetscSectionAddDof(s,p,2) ;
> > }
> > Then, at shared 'p', section 's' has dof=3?
>
> Sections are "local" objects that are tied together by an SF that
> describes point ownership.
>
> So I think that the only thing you need is that if two processes set a dof
> on what is globally the same point, they should agree on how many dofs live
> there.
>
> I wonder if PetscSectionAddDof (etc...) should be marked as logically
> collective.
>
>
> Right now, what you want is handled by creating a global Section from a
> local Section:
>
>
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PetscSection/PetscSectionCreateGlobalSection.html
>
> The global Section contains global offsets for all local points, and the
> ghost points have negative offsets.
>
> There is no facility for combining dofs. The idea is that you know the
> number of dofs on each local point. If you change that
> interpretation, you could easily use SF to add them up and broadcast the
> sum.
>
>   Thanks,
>
>      Matt
>
>
> Lawrence
>
> --
> 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/>
>


-- 
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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