[petsc-users] Scale the diagonal entries of a Mat

[Hui Zhang]

Should MatZeroRowsColumnsLocalIS also be done before MatAssembly?

I used MatZeroRows one year ago.  At that time, petsc asked me the Mat must have already been assembled. 

On Feb 23, 2013, at 2:37 PM, Jed Brown wrote:

> (You should really not use penalties, but if you insist on doing it, you should implement it the way I describe. There is no point assembling before imposing boundary conditions.)
> 
> On Feb 23, 2013 7:32 AM, "Jed Brown" <jedbrown at mcs.anl.gov> wrote:
> You should really use MatSetValuesLocal with ADD_VALUES *before* assembling.
> 
> On Feb 23, 2013 7:28 AM, "Hui Zhang" <mike.hui.zhang at hotmail.com> wrote:
> 
> On Feb 23, 2013, at 1:37 PM, Matthew Knepley wrote:
> 
> > On Sat, Feb 23, 2013 at 7:35 AM, Hui Zhang <mike.hui.zhang at hotmail.com> wrote:
> >
> > On Feb 23, 2013, at 1:32 PM, Matthew Knepley wrote:
> >
> > > On Sat, Feb 23, 2013 at 7:29 AM, Hui Zhang <mike.hui.zhang at hotmail.com> wrote:
> > > I want to implement diagonal penalty method for enforcing the Dirichlet boundary conditions.  That is, the diagonal entries corresponding to Dirichlet boundary are going to be scaled by a large number.
> > >
> > > What is the easiest way to do this?  Thanks!
> > >
> > > http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/MatDiagonalScale.html
> >
> > But it seems not what I want.  That routine scales all the entries of a mat.  I want to scale only the diagonal entries.
> >
> > Then use http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/Mat/MatDiagonalSet.html
> 
> I want to do the following on an already assembled matrix (FINAL_ASSEMBLY).
> 
> MatGetLocalSubMatrix .. to get 'submat',
> MatGetDiagonal .. to get diagonal vector 'diag',
> VecScale .. to scale 'diag'
> MatDiagonalSet .. to set scaled 'diag' to 'submat'
> MatRestoreLocalSubMatrix .. to restore 'submat'
> 
> After the above process, do I need MatAssemblyBegin/End again?  I do not know why the penalty method is bad.  Maybe because the bad conditioning?  I just want to try out and compare with MatZeroRowsColumns (lift and remove) method.
> 
> >
> > As Jed says, you really really do not want to do this.
> >
> >   Matt
> >
> > >
> > >   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
> >
> >
> >
> >
> > --
> > 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
>