[boost] [mpi] Automatically Cascading/Nested Reduce?

Subject: [boost] [mpi] Automatically Cascading/Nested Reduce?
From: Dean Michael Berris (mikhailberis_at_[hidden])
Date: 2009-04-20 13:14:11

Hi Guys,

I'm writing quite a number of parallel-distributed applications in the
day job, and I'm missing a facility which allows for "automagically"
nesting my reductions. The reduction strategy I'm looking for is one
of a networked approach where (if you can imagine a tree):

0
|-1
|  |-2
|  |-3
|
|-4
  |-5
  |-6

Nodes 2 and 3 send their data to node 1, node 5 and 6 send their data
to node 4, and eventually node 1 and 4 send their (reduced) data to
node 0. My idea is that it should be simple to implement this without
having to go through too much trouble with communicators -- or that it
could be automagically done by hiding the communicator splits in the
special reduce implementation.

Would something like this be best implemented within Boost.MPI? Or is
there a way of implementing this by composing the functionality
already available in Boost.MPI?

The reason I ask is that sometimes the reduction step dominates the
amount of time the application spends especially when you have quite a
number of nodes (around 90ish). Being able to parallelize (or nest)
the reduction would definitely help, but the cost of supporting that
routine over a number of applications seems to warrant a special
implementation of the Boost.MPI reduce.

TIA

-- 
Dean Michael Berris | Software Engineer, Friendster, Inc.
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