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

[Barry Smith]

> On Jan 11, 2022, at 9:51 PM, Matthew Knepley <knepley at gmail.com> wrote:
> 
> On Tue, Jan 11, 2022 at 3:31 PM Barry Smith <bsmith at petsc.dev <mailto: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.

  This is not cumbersome at all. It is a simple natural way to pass around coordinates to PC's and, when possible, their children. 

  Barry

  Note that we could also have a DMGetCoordinates() that pulled coordinates from a DM (that happended to have them) in this form associated with the PC and the PC could call it to get the coordinates and use them as needed. But this simple PCSetCoordinates() is a nice complement to that approach.

> 
>   Thanks,
> 
>     Matt
>  
>   Barry
> 
> 
> > On Jan 11, 2022, at 11:58 AM, Nicolás Barnafi <nabw91 at gmail.com <mailto: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/>

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