[petsc-dev] Ghost update in VecAssembly?

[Jed Brown]

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.

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