dolfin team mailing list archive

[Ola Skavhaug <skavhaug@xxxxxxxxx>]

Anders Logg skrev den 21/08-2008 følgende:
> On Thu, Aug 21, 2008 at 09:10:03AM +0200, Niclas Jansson wrote:
> > Anders Logg wrote:
> > > On Wed, Aug 20, 2008 at 06:17:30PM +0200, Niclas Jansson wrote:
> > > 
> > >>>> Stage 2 seems to involve a lot of communication, with small messages.
> > >>>> I think it would be more efficient if the stage were reorganized such
> > >>>> that all messages could be exchanged "at once", in a couple of larger
> > >>>> messages.
> > >>> That would be nice. I'm very open to suggestions.
> > >> If understand the {T, S, F} overlap correctly, a facet could be globally
> > >> identified by the value of F(facet).
> > > 
> > > No, F(facet) would be the local number of the facet in subdomain S(facet).
> > > 
> > >> If so, one suggestion is to buffer N_i and F(facet) in 0...p-1 buffers
> > >> (one for each processor) and exchange these during stage 2.
> > >>
> > >> -- stage 1
> > >>
> > >>   for each facet  f \in T
> > >>     j = S_i(f)
> > >>     if j > i
> > >>
> > >>         -- calculate dof N_i
> > >>
> > >>         buffer[S_i(f)].add(N_i)
> > >>         buffer[S_i(f)].add(F_i(f))
> > >>     end
> > >>   end
> > >>
> > >>
> > >> -- stage 2
> > >>
> > >> -- Exchange shared dofs with fancy MPI_Allgatherv or a lookalike
> > >> -- MPI_SendRecv loop.
> > >>
> > >>    for j = 1 to j = (num processors - 1)
> > >>       src = (rank - j + num processors) % num processors
> > >>       dest = (rank + j) % num processors
> > >>
> > >>       MPI_SendRecv(dest, buffer[dest], src, recv_buffer)
> > >>
> > >>       for i = 0 to sizeof(recv_buffer), i += 2
> > >>          --update facet recv_buff(i+1) with dof value in  recv_buff(i)
> > >>       end
> > >>
> > >>    end
> > > 
> > > I didn't look at this in detail (yet). Is it still valid with the
> > > above interpretation of F(facet)?
> > > 
> > 
> > Yes, I think so.
> 
> I think I understand your point, but I don't understand the details
> of your code.
> 
> The mapping N_i is an auxiliary global-to-global mapping, which maps
> the global dofs on a local mesh to global dofs on the global mesh. It
> has a meaning only on each local mesh. What we want to communicate is
> the stuff in M_i.
> 
> Should we try to implement this? It will essentially be Algorithm 5++
> (Algorithm 5 with your improvements). So we don't store a global
> numbering of mesh entities but instead compute a global dof map in
> parallel. And we store the overlap as MeshData in some way (a set of
> MeshFunctions attached to each local mesh). I'm very open to which set
> of MeshFunctions we will need, either just S, F or additional data we
> might need.
> 
> Other opinions? Garth? Ola?

Sounds good, lets do it. One problem though; if we want to store the facet
overlap in MeshFunctions as MeshData, the numbering of the facets is implicit,
depending on the algorithm used in DOLFIN.

Ola
 
> -- 
> Anders



> _______________________________________________
> DOLFIN-dev mailing list
> DOLFIN-dev@xxxxxxxxxx
> http://www.fenics.org/mailman/listinfo/dolfin-dev