[mpiwg-coll] backward communication in neighborhood collectives

[Junchao Zhang]

Dan,
  Using your approach, I have to specify zero messages in arguments to
MPI_NEIGHBOR_ALLTOALLV,
because I only do communication in one direction in one time.  Also, in my
graph, there may have edges A->B and B->A. They may have different weights.
Using your approach, it is quite confusing.
  One uses MPI_NEIGHBOR_ALLTOALLV to have better performance.  We have to
keep it in mind.

--Junchao Zhang

On Mon, Jul 23, 2018 at 9:59 AM, HOLMES Daniel <d.holmes at epcc.ed.ac.uk>
wrote:

> Hi Junchao Zhang,
>
> You are correct - the neighbourhood collectives send messages to (and
> receive messages from) only neighbouring MPI processes, as defined by the
> virtual topology. However, it is possible to use point-to-point,
> single-sided, or normal collective functions to send (and receive) messages
> between *any* MPI processes in a communicator, irrespective of the
> presence/structure of a virtual topology.
>
> For your use-case (if I understand it correctly), you should specify a
> symmetric graph - each process specifies to MPI_DIST_GRAPH_CREATE_ADJACENT
> all the processes it wishes to communicate with (both sending and
> receiving) during subsequent calls to MPI_NEIGHBOR_ALLTOALLV.
>
> 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 23 Jul 2018, at 15:45, Junchao Zhang <jczhang at mcs.anl.gov> 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.
>
> --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
>>>
>>> _______________________________________________
>>> mpiwg-coll mailing list
>>> mpiwg-coll at lists.mpi-forum.org
>>> https://lists.mpi-forum.org/mailman/listinfo/mpiwg-coll
>>>
>>>
>>
>>
>> --
>> Tony Skjellum, PhD
>> RunTime Computing Solutions, LLC
>> tony at runtimecomputing.com
>> direct: +1-205-918-7514
>> cell: +1-205-807-4968
>> _______________________________________________
>> mpiwg-coll mailing list
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>> https://lists.mpi-forum.org/mailman/listinfo/mpiwg-coll
>>
>>
>>
>> The University of Edinburgh is a charitable body, registered in
>> Scotland, with registration number SC005336.
>>
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>>
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