Anders Logg wrote:
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.
if j > i the processor is responsible for creating M_i for the shared
facet. The newly created M_i is placed in the send buffer for the
subdomain S_f(f), together with the local facet number in that subdomain.
So the send buffers contains tuples {M_i, F_i(f)}, since there is one
buffer for each subdomain, one could be sure that F_i(f) is valid on the
receiving processor.
Instead of iterating over all processors and facets in stage 2, each
processor receives a set of tuples (for all shared facets) from each
processor. These could then be used to identify the local facet (since
F_i(f) is the local facet number) and assign the dofs, which, if I
understand everything correctly is obtained from M_i.
One modification to the above algorithm, I think it's easier if the
tuples are stored as {F_i(f), M_i}. Since M_i could be a long list of
dofs. So the update loop would be something similar to
for i = 0 to size of recv_buff , i +=(number of dofs on each facet + 1)
local facet f = recv_buff(i)
for each facet on f, loop counter j
assign recv_buff( (i+1) + j) ) to facet dof j
end
end
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.
I see, then it should be M_i in the outlined code.
Niclas
