[mpiwg-coll] backward communication in neighborhood collectives

[Junchao Zhang]

Hello, Dan,
 I am interested in MPI_Neighbor_alltoallv. From its arguments, I do not
see one can send a message to a neighbor not specified in the graph created
by for example, MPI_Dist_graph_create_adjacent.

--Junchao Zhang

On Mon, Jul 23, 2018 at 5:51 AM, HOLMES Daniel <d.holmes at epcc.ed.ac.uk>
wrote:

> 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.
> —
> Dr Daniel Holmes PhD
> Applications Consultant in HPC Research
> d.holmes at epcc.ed.ac.uk
> Phone: +44 (0) 131 651 3465
> Mobile: +44 (0) 7940 524 088
> Address: Room 3415, JCMB, The King’s Buildings, Edinburgh, EH9 3FD
> —
> The University of Edinburgh is a charitable body, registered in Scotland,
> with registration number SC005336.
> —
>
> On 15 Jul 2018, at 10:42, Anthony Skjellum <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>
> 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
> RunTime Computing Solutions, LLC
> tony at runtimecomputing.com
> direct: +1-205-918-7514
> cell: +1-205-807-4968
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>
>
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> Scotland, with registration number SC005336.
>
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