[petsc-dev] MatAssembly and VecAssembly

[Kong, Fande]

On Fri, Oct 20, 2017 at 4:24 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Oct 20, 2017, at 12:17 PM, Kong, Fande <fande.kong at inl.gov> wrote:
> >
> > How about to do  a global check (MPI_Allreduce)?
>
>    Kill you for large number of processes.
>
> > If we do not set values at all and the matrix is already assembled, we
> just return without doing anything?
> >
> > How expensive, in the current implementation,  to call
> MatAssemblyBegin/End if there are no any stashed data? Is it so cheap that
> we can just ignore it?
>
>    It requires at least one global reduction.
>

So, this global reduction is cheaper than MPI_Allreduce?  Or they are
similar.




> >
> > I am asking because we call MatAssemblyBegin/End so often in MOOSE. I
> want to make sure this is not going to bring up any performance issue.
>
>   You need to check each use in MOOSE and see WHY it is being called. If
> no reason then don't call.
>

Good point.

Fande,



>
>   Barry
>
> >
> >
> > Fande,
> >
> > On Fri, Oct 20, 2017 at 11:08 AM, Barry Smith <bsmith at mcs.anl.gov>
> wrote:
> >
> >   One process sets a value in the matrix, the others do not. They all
> call MatAssemblyBegin(). Some processes will skip the assembly and hence
> the code will hang.
> >
> >
> > > On Oct 20, 2017, at 12:03 PM, Kong, Fande <fande.kong at inl.gov> wrote:
> > >
> > > Hi All,
> > >
> > > In Mat/VecAssemblyBegin/End, why we do not check if or not mat/vec is
> assembled. If mat/vec is already assembled, should we just return without
> doing anything?
> > >
> > > I think we have some particular reasons not to check if the matrix is
> assembled. I honestly do not know why.
> > >
> > > Thanks,
> > >
> > > Fande,
> >
> >
>
>
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