[petsc-users] getting the near nullspace from PCSetCoordinates

[Jed Brown]

Mark Adams <mfadams at lbl.gov> writes:

> On Mon, Jan 2, 2017 at 11:47 AM, Jed Brown <jed at jedbrown.org> wrote:
>
>> Jeremy Theler <jeremy at seamplex.com> writes:
>>
>> > Hi all
>> >
>> > I want to check that the near nullspace I provide to GAMG gives "almost
>> > null vectors" when multiplying each vector in the near nullspace against
>> > the matrix problem.
>> >
>> > This way I can check that the unknown ordering I am using is consistent,
>> > for example using by MatNullSpaceCreateRigidBody() or by computing the
>> > nullspace by myself.
>>
>> Please use that and MatSetNearNullSpace().  It composes properly and you
>> can check everything.
>>
>> PCSetCoordinates() happens to do double-duty for aggregation-based
>> methods, but outside of semi-geometric methods, it is just ugly code
>> duplication and makes assumptions that may be inappropriate (like
>> elasticity with an interpolatory basis).
>
>
> Yes, PCSetCoordinates is an old interface that is essentially deprecated.
> Maybe we should officially deprecated this.

I think we should officially deprecate it, but perhaps make something
more general available as a Mat function (since some algorithms may use
coordinates directly).  (Needing to dig up a PC to provide problem (as
opposed to configuration) information is bad style.)
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