On Mon, Jul 23, 2018 at 7:45 AM, Junchao Zhang <[email protected]> wrote:

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.

Why would you expect this? The whole point of graph communicators is to specify the communication topology so that it can be optimized. If you want to communicate with another ranks, use the parent communicator that supports communication to all ranks. Jeff

--Junchao Zhang

On Mon, Jul 23, 2018 at 5:51 AM, HOLMES Daniel <[email protected]> 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 [email protected] 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 <[email protected]> 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 <[email protected]> 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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