[petsc-dev] Composing PetscSections for FieldSplit

[Michael Lange]

Hi Matt,

We are  trying to implement fieldsplit for mixed function spaces in 
Firedrake. For this we want to compose a set of sections with one field 
each into one section with several fields. The is to add a 
PetscSectionSetField() function and then use this to build a container 
section with something like:

sec_offset = 0
msec = PETSc.Section().create()
for i, s in enumerate(spaces):
     sec = shift_offsets(s.clone(), sec_offset)
     msec.setField(i, sec)
     sec_offset += s.getStorageSize()

We have tried setting this as the DefaultSection in our DM and then 
passing this to SNES via SNESSetDM(), but the problem we encountered is 
that we get an empty global section, because 
PetscSectionCreateGlobalSection() can currently not handle multiple fields.

Do you agree with this approach? And if so, can we make 
PetscSectionCreateGlobalSection() aware of fields?

Thanks,

Michael