[petsc-dev] Ghost update in VecAssembly?

[Matthew Knepley]

On Tue, Mar 1, 2016 at 10:36 AM, Jed Brown <jed at jedbrown.org> wrote:

> Matthew Knepley <knepley at gmail.com> writes:
>
> > On Tue, Mar 1, 2016 at 10:19 AM, Jed Brown <jed at jedbrown.org> wrote:
> >
> >> Matthew Knepley <knepley at gmail.com> writes:
> >> > I was confusing the VecGhostUpdate() above which puts local values in
> the
> >> > global vector
> >> > with the update which would refresh the local portion. The former
> update
> >> > should be called
> >> > automatically during assembly, which already happens in the
> >> > FormFunctionLocals,
> >>
> >> Uh, no, and they don't use VecGhost...
> >
> >
> > Yes, it most certainly does happen when FormFunctionLocal is
> > used. PETSc calls Assembly, at least in the Plex variant. Of course,
> > they do not use VecGhost.
>
> SNESComputeFunction_DMDA:
>
>   switch (dmdasnes->residuallocalimode) {
>   case INSERT_VALUES: {
>     ierr = DMDAVecGetArray(dm,F,&f);CHKERRQ(ierr);
>     ierr = PetscLogEventBegin(SNES_FunctionEval,snes,X,F,0);CHKERRQ(ierr);
>     CHKMEMQ;
>     ierr =
> (*dmdasnes->residuallocal)(&info,x,f,dmdasnes->residuallocalctx);CHKERRQ(ierr);
>     CHKMEMQ;
>     ierr = PetscLogEventEnd(SNES_FunctionEval,snes,X,F,0);CHKERRQ(ierr);
>     ierr = DMDAVecRestoreArray(dm,F,&f);CHKERRQ(ierr);
>   } break;
>   case ADD_VALUES: {
>     Vec Floc;
>     ierr = DMGetLocalVector(dm,&Floc);CHKERRQ(ierr);
>     ierr = VecZeroEntries(Floc);CHKERRQ(ierr);
>     ierr = DMDAVecGetArray(dm,Floc,&f);CHKERRQ(ierr);
>     ierr = PetscLogEventBegin(SNES_FunctionEval,snes,X,F,0);CHKERRQ(ierr);
>     CHKMEMQ;
>     ierr =
> (*dmdasnes->residuallocal)(&info,x,f,dmdasnes->residuallocalctx);CHKERRQ(ierr);
>     CHKMEMQ;
>     ierr = PetscLogEventEnd(SNES_FunctionEval,snes,X,F,0);CHKERRQ(ierr);
>     ierr = DMDAVecRestoreArray(dm,Floc,&f);CHKERRQ(ierr);
>     ierr = VecZeroEntries(F);CHKERRQ(ierr);
>     ierr = DMLocalToGlobalBegin(dm,Floc,ADD_VALUES,F);CHKERRQ(ierr);
>     ierr = DMLocalToGlobalEnd(dm,Floc,ADD_VALUES,F);CHKERRQ(ierr);
>     ierr = DMRestoreLocalVector(dm,&Floc);CHKERRQ(ierr);
>   } break;
>
> See how this uses DMLocalToGlobal and there is no VecAssembly.
>

Yes, no Assembly. We only call it for matrices. The asymmetry is a little
strange.


> src/snes/utils/dmplexsnes.c is far more confusing, but still contains no
> VecAssembly.
>

It does not seem confusing to me.

   Matt

-- 
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
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