[petsc-users] PETSCFE_CLASSID/PETSCVF_CLASSID

[Jacob Faibussowitsch]

Nicolas,

> I am trying to understand how the class IDs of a DM field are set

Class ID’s are unique (internal) identifiers that every type of object of a petsc-created class shares to identify them. This is especially useful when these objects are passed around semi-opaquely by casting to PetscObject. Think of it as similar to c++ typeid https://en.cppreference.com/w/cpp/language/typeid <https://en.cppreference.com/w/cpp/language/typeid> 

> ierr = PetscFECreateDefault(PetscObjectComm((PetscObject) dmGrad), dim, coordDim, PETSC_TRUE, NULL, -1, &feGrad);CHKERRQ(ierr);
> ierr = PetscDSSetDiscretization(probGrad, f, (PetscObject) feGrad);CHKERRQ(ierr);
> ierr = PetscFEDestroy(&feGrad);CHKERRQ(ierr);
> 
> Yet, when I call DMPlexComputeGradientClementInterpolant, I get the following error:
> [0]PETSC ERROR: Unknown discretization type for field 0

This function walks through all of the fields you have added to the DM and performs a sanity check (using the classics) to determine whether they are all the correct object. 

It seems like you’ve missed a step here. Have you called DMAddField()/DMSetField() to associate your PetscFE with your plex? Note that 

> ierr = PetscFECreateDefault(PetscObjectComm((PetscObject) dmGrad), dim, coordDim, PETSC_TRUE, NULL, -1, &feGrad);CHKERRQ(ierr);

Does not directly tie the feGrad to your dmGrad, rather just gives it the same MPI_Comm. 

Best regards,

Jacob Faibussowitsch
(Jacob Fai - booss - oh - vitch)
Cell: (312) 694-3391

> On Dec 11, 2020, at 11:02, Nicolas Barral <nicolas.barral at math.u-bordeaux.fr> wrote:
> 
> Hi all (and probably more specifically Matt ?)
> 
> I am trying to understand how the class IDs of a DM field are set, and can't find it in the documentation.
> 
> A little background, I am mimicking SNES/utils/dmadapt.c/DMAdaptorAdapt_Sequence_Private for a specific case (I'm trying to build the same kind of metric from a single sensor field, without all the SNES layer).
> 
> I need to compute the gradient of the sensor field, using DMPlexComputeGradientClementInterpolant, for which I create a DM, to which I associate a PetscFE, a DS, like in existing code:
> 
> PetscFE  feGrad
> PetscDS  probGrad
> 
> ierr = PetscFECreateDefault(PetscObjectComm((PetscObject) dmGrad), dim, coordDim, PETSC_TRUE, NULL, -1, &feGrad);CHKERRQ(ierr);
> ierr = PetscDSSetDiscretization(probGrad, f, (PetscObject) feGrad);CHKERRQ(ierr);
> ierr = PetscFEDestroy(&feGrad);CHKERRQ(ierr);
> 
> Yet, when I call DMPlexComputeGradientClementInterpolant, I get the following error:
> [0]PETSC ERROR: Unknown discretization type for field 0
> 
> I don't fully understand what all these objects are (FE, DS and Field), and how they are related, where would that be documented ?
> And what else do I need to do to make my example work ?
> 
> Thanks
> 
> -- 
> Nicolas

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