[petsc-users] PCSetCoordinates does not set coordinates of sub PC (fieldsplit) objects

[Matthew Knepley]

On Tue, Jan 11, 2022 at 3:31 PM Barry Smith <bsmith at petsc.dev> wrote:

>
>   Nicolas,
>
>     For "simple" PCFIELDSPLIT it is possible to pass down the attached
> coordinate information. By simple I mean where the splitting is done by
> fields and not by general lists of IS (where one does not have enough
> information to know what the coordinates would mean to the subPCS).
>
>    Look in fieldsplit.c PCFieldSplitSetFields_FieldSplit() where it does
> the KSPCreate(). I think you can do a KSPGetPC() on that ksp and
> PCSetCoordinates on that PC to supply the coordinates to the subPC. In the
> function PCFieldSplitSetIS_FieldSplit() you can also attach the coordinates
> to the subPCs IF defaultsplit is true.
>
>    Sadly this is not the full story. The outer PC will not have any
> coordinates because calling PCSetCoordinates on a PCFIELDSPLIT does nothing
> since fieldsplit doesn't handle coordinates. So you need to do more, you
> need to provide a PCSetCoordinates_FieldSplit() that saves the coordinates
> in new entries in the PC_FieldSplit struct and then in
> PCFieldSplitSetFields_FieldSplit() you need to access those saved values
> and pass them into the PCSetCoordinates() that you call on the subPCs. Once
> you write
> PCSetCoordinates_FieldSplit()  you need to call
>
>   ierr =
> PetscObjectComposeFunction((PetscObject)pc,"PCSetCoordinates_C",PCSetCoordinates_FieldSplit);CHKERRQ(ierr);
>
>
> inside PCCreate_FieldSplit().
>
>    Any questions just let us know.
>

I will add "Why is this so cumbersome?". This is a workaround in order to
get geometric information into GAMG. It should really be
PCGAMGSetCoordinates(), which
are used to calculate the rigid body modes, and assume a bunch of stuff
about the coordinate space. This would not help you, because it would still
force you to pull
out the correct subPC. The "right" way now to give geometric information to
a TS/SNES/KSP/PC is through a DM, which are passed down through
PCFIELDSPLIT,
PCPATCH, etc. However they are heavier weight than just some coordinates.

  Thanks,

    Matt


>   Barry
>
>
> > On Jan 11, 2022, at 11:58 AM, Nicolás Barnafi <nabw91 at gmail.com> wrote:
> >
> > Dear community,
> >
> > I am working on a block preconditioner, where one of the blocks uses
> HYPRE's AMS. As it requires the coordinates of the dofs, I have done so to
> the PC object. I expected the coordinates to be inherited down to the
> subblocks, is this not the case? (it seems so as I couldn't find a
> specialized FIELDSPLIT SetCoordinates function).
> >
> > If this feature is missing, please give me some hints on where to add
> the missing function, I would gladly do it. If not, please let me know why
> it was dismissed, in order to do things the hard way [as in hard-coded ;)].
> >
> > Kind regards,
> > Nicolas
>
>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20220111/02d93f88/attachment-0001.html>