[petsc-users] Understanding DMPlexDistribute overlap

[Michael Lange]

Hi Francesco,

Your description is almost correct: the overlap defines the topological 
depth of shared entities as counted in "neighboring cells", where a cell 
counts as a neighbor of an owned cell according to the defined adjacency 
style. So for overlap=0 only facets, edges and vertices may be shared 
along the partition boundary, whereas for overlap=1 you can expect one 
additional "layer" of cells around each partition (the partitioning is 
done based on cell connectivity). For second neighbors, however, you 
need overlap=2. And yes, there is conceptually no upper bound on the 
overlap.

Hope this helps,

Michael


On 05/04/17 10:27, Francesco Caimmi wrote:
> Dear all,
>
> I was playing with DMPlex objects and I was trying to exactly figure out what
> the `overlap` parameter in DMPlexDistribute does.
>
>  From the tutorial "Flexible, Scalable Mesh and Data Management
> using PETSc DMPlex" (slide 10) and from the work by Knepley et al.
> "Unstructured Overlapping Mesh Distribution in Parallel" I somehow got the
> idea that it should control the "depth" of the mesh overlap.
> That is, given the partition boundary, if overlay is set to 0 only the
> entities adjacent (in the DMPlex topological sense and with the "style" defined
> by the AdjacencyUse routines) to entities at the boundary are shared, if
> overlay is 1 the first and the second neighbors (always in the DMPlex
> topological sense) are shared and so on, up to the point were we have a full
> duplicate of the mesh on each process (i.e. there is no upper bound on
> `overlap`).
>
> Is this correct or am I -totally- misunderstanding the meaning of the
> parameter?
>
> I am asking this because I see some behavior I cannot explain at varying the
> value of the overlap, but before going into the details I would like to be
> sure to understand  exactly what the overlap parameter is supposed to do.
>
> Many thanks,