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Subject: Re: [boost] [mpi] Automatically Cascading/Nested Reduce?
From: John Phillips (phillips_at_[hidden])
Date: 2009-04-20 19:46:02

Dean Michael Berris wrote:
> 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.


   What MPI implementation are you using?

   I ask because I believe that at least some of them already use
optimizations of this sort when implementing reduce. That is really more
the right place for a communications scheme of this sort. Boost.MPI just
uses the facilities for this that are provided by MPI itself. In
principle, the MPI implementation should do the reduce in as efficient a
way as it can. What does yours currently do?


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