[petsc-users] PetscFE questions

[Lawrence Mitchell]

> On 21 Oct 2016, at 08:26, Julian Andrej <juan at tf.uni-kiel.de> wrote:
> 
> On Thu, Oct 20, 2016 at 5:18 PM, Matthew Knepley <knepley at gmail.com> wrote:
>> On Thu, Oct 20, 2016 at 9:42 AM, Julian Andrej <juan at tf.uni-kiel.de> wrote:
>>> 
>>> Thanks for the suggestion. I guess DMCreateSubDM can work, but is
>>> cumbersome to handle for the normal solution process since the mass
>>> matrix for example is not a seperate field.
>> 
>> 
>> I did not understand what you meant by "parts of the physics". If you just
>> want to make a different operator, then swap out the PetscDS from the DM.
>> That holds the pointwise functions and discretizations.
>> 
> 
> Yes, its basically a different operator! Thats a really smart design,
> i can just create different PetscDS objects and stick them in to
> assemble the operator.
> 
>  /* Assemble mass operator */
>  DMSetDS(dm, ds_mass);
>  DMPlexSNESComputeJacobianFEM(dm, dummy, ctx->M, ctx->M, NULL);
>  /* Assemble laplacian operator */
>  DMSetDS(dm, ds_laplacian);
>  DMPlexSNESComputeJacobianFEM(dm, dummy, ctx->J, ctx->J, NULL);
> 
> There is one thing that bothers me just a bit. Everytime you call
> DMSetDS the old PetscDS object is destroyed and you have to reacreate
> the object in case you want to reassemble that operator.
> 
> src/dm/interface/dm.c:3889:  ierr = PetscDSDestroy(&dm->prob);CHKERRQ(ierr);

All objects in PETSc are refcounted.  So this just drops the reference that the DM is holding to the DS.  As long as you're still holding a reference in your code (you haven't called PetscDSDestroy) then this does not actually deallocate the DS, just decrements the refcount.

Lawrence