[petsc-dev] Attaching a near null space to an IS

[Matthew Knepley]

On Sun, May 6, 2012 at 7:15 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> Matt introduced this concept, he says the IS is a better place to attach
> things.
>
> http://petsc.cs.iit.edu/petsc/petsc-dev/rev/2ad289ac99e0
>
> I don't understand why the IS is better (because it's mostly immutable?).
> I'm worried that putting it there is going to be fragile because the near
> null space is not a property of an IS at all.
>

Its not a property of your matrix either, or you would not need me to tell
you. Its a property of the operator. The operator
is defined by the DM. The near null space is actually a property of a
suboperator, defined by the DM using a field (we are
not allowing arbitrary divisions). The representation of a field in PETSc
is an IS (
http://www.mcs.anl.gov/petsc/petsc-dev/docs/manualpages/DM/DMCreateFieldIS.html)
so it makes sense to attach field information to the IS. Moreover, it
makes a hell of a lat more sense to attach an auxiliary operator (like L_p)
to this IS than to a matrix.

Furthermore, this scheme is completely workable in a nested context. The
user can specify the IS, or pull out the DM IS
and play with it, without a bunch of cumbersome copies hanging around that
we do not want and can't destroy. That is what
would happen with persistent submatrices. Lastly, I am running this for
PyLith and it works great.

   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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