dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

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)?

-- 
Anders

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