[Libmesh-users] Parallel solve system on partitioned mesh

[Stephan H. <inf...@st...>]

Hello,

first of all: I'm new to libmesh and therefore I have still difficulties
to fully understand how certain things work with libmesh. So please be
patient with me ;-)
I wrote a code to compute displacements and drilling moments for flat
shell elements with FEM. It imports a mesh file (xda or msh), assembles
the system matrix and RHS, solves the system and exports the results as
vtk files.
When I run my program with one processor it all works fine and the
displacements are correct. The problem occurs with more than one processor.
Simple example: Plane mesh (e.g. 8x8 quadrilaterals). Left edge is
clamped, the force is applied at the right edge and only at this edge.
If I run this example with e.g. 2 processors the partitioning cut halves
the mesh in a left and a right part. The result looks as follows: The
left part has no displacement at all, while the right part has a huge
displacement.
I studied your example codes and couldn't see any special code fragments
that is needed for a parallelized version of the code, so I thought:
'Hey, libmesh seems to handle the parallelization internally - that's
great!'. But when I look at my results; in my opinion every processor
solves its own partial system with its own boundary condition, mesh
nodes and RHS and produces its own solution - so no communication or
exchange between the processes at all...

Now to my questions: Do I need additional configurations in my code to
make it work (at the LinearImplicitSystem, the mesh, the system matrix,
etc.)? Additional MPI code to exchange values/vectors/matrices by my
own? Libmesh seems to use MPI communication extensively internally and
it's difficult for me right now to see where the problem is. Perhaps you
can give me a hint to be back on track soon.

Thanking you in anticipation for your help.

Bye,
Stephan Herb