[mpiwg-coll] backward communication in neighborhood collectives

[HOLMES Daniel]

Hi Junchao Zhang,

My understanding of the current API for MPI-3.1 is that:

1) the virtual topology does not actually restrict communication via the communicator to the edges specified in the topology - messages can be sent along any edge in either direction, and even between pairs of processes for which no edge was specified.

2) the virtual topology can be specified as a symmetric graph - for every ‘forward’ edge (e.g. from A to B), the ‘backward’ edge (i.e. from B to A) can be included as well.

3) there is already language in the MPI Standard regarding how MPI handles symmetric and non-symmetric graph topologies for neighbourhood collective operations.

Thus, there is no need to create two distributed graph topology communicators to achieve ‘forward and backward communication along the edges’.

Cheers,
Dan.
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Dr Daniel Holmes PhD
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On 15 Jul 2018, at 10:42, Anthony Skjellum <tony at runtimecomputing.com<mailto:tony at runtimecomputing.com>> wrote:

Hi, I just saw this.

We definitely need to consider this concern.

I also need to go review the APIs.

Thanks,
Tony


On Sat, Jul 14, 2018 at 12:27 AM, Junchao Zhang <jczhang at mcs.anl.gov<mailto:jczhang at mcs.anl.gov>> wrote:
 I want to try MPI neighborhood collectives. I have a communication graph and want to do both forward and backward communication along the edges. With current APIs, it looks I need to create two comm_dist_graphs. It is a waste since MPI implementations can do similar optimizations for the two.
 Should MPI support this scenario?  Thanks.

--Junchao Zhang

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Tony Skjellum, PhD
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