[firedrake] Firedrake/DMPlex parallel restriction: one d.o.f. per process

[Ed Bueler]

Nice!  Works for me.  E.g. example below shows zero 2-cells on some
processes.  I'll fix this aspect of chapter 13 in my (soon) book.

Ed

$ mpiexec -n 10 python fish.py -dm_view
DM Object: Parallel Mesh 10 MPI processes
  type: plex
Parallel Mesh in 2 dimensions:
  0-cells: 3 0 3 3 3 3 3 0 3 3
  1-cells: 3 0 3 3 3 3 3 0 3 3
  2-cells: 1 0 1 1 1 1 1 0 1 1
Labels:
  depth: 3 strata with value/size (0 (3), 1 (3), 2 (1))
  Face Sets: 0 strata with value/size ()
  boundary_faces: 0 strata with value/size ()
  exterior_facets: 0 strata with value/size ()
  interior_facets: 1 strata with value/size (1 (3))
done on 3 x 3 grid with P^1 elements:
  error |u-uexact|_inf = 3.365e-03, |u-uexact|_h = 1.190e-03



On Wed, Dec 18, 2019 at 2:41 AM Lawrence Mitchell <wencel at gmail.com> wrote:

>
>
> > On 17 Dec 2019, at 19:46, Ed Bueler <elbueler at alaska.edu> wrote:
> >
> > Firedrake devs --
> >
> > This is substantially a matter of curiosity, but I figure I am missing
> something I should understand.
> >
> > Namely, parallel runs will generate "RuntimeError: Mesh must have at
> least one cell on every process" if you attempt to have less.  My
> understanding is that this restriction is not intrinsic to PETSc DMPlex,
> though the restriction does apply to DMDA.  I know that Firedrake uses
> separate DMPlex for the mesh and for the data layout.  What is the basic
> story here?
>
> The historical story was that we previously relied on having one element
> on every process to be able to determine the cell shape in parallel without
> communicating. We now do it a different (more robust) way.
>
> As Koki points out, in current firedrake this restriction is lifted.
>
> Cheers,
>
> Lawrence



-- 
Ed Bueler
Dept of Mathematics and Statistics
University of Alaska Fairbanks
Fairbanks, AK 99775-6660
306C Chapman
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