[Libmesh-devel] Refinement flag assert trip when using GMG + AMR

[Boris B. <bor...@bu...>]

Hello all,

Ive been looking into using GMG with AMR on an L-shaped domain problem and
am having issues uniformly_coarsening during the GMG setup phase after I've
refined some elements on the first AMR step in both serial and parallel
situations. I am using elem_fraction AMR flagging and am setting a small
refinement_percentage and have set coarsening_percentage to 0 to keep
things simple.

Specifically in this 2 level GMG scenario, I  end up tripping the assert on
elem_refinement.C:157 i.e.
libmesh_assert_equal_to (mychild.refinement_flag(), Elem::COARSEN) while
trying to coarsen an element, at this point mychild.refinement_flag() ==
coarsen_inactive.

When I hit the assert, the element in question that's being coarsened is
actually already inactive, level 0 and having a coarsen_inactive refinement
flag, and has children with refinement flags: c[0]: inactive, c[1]:
coarsen_inactive, (both of which are inactive) while c[2], c[3]: are active
with a flag == coarsen.

While the c[0,2,3] children have no children themselves, c[1] does, and has
all its children as active, but with a refinement flag == coarsen.

In short, I've been trying to understand how the refinement flags got into
such a state but haven't had much luck so far and wondering if there were
any clues about how I could have ended up in such a situation while trying
to do the uniform coarsening.

Thanks in advance for any info provided,
- Boris