[mpiwg-coll] backward communication in neighborhood collectives

[Junchao Zhang]

Jeff,
 No. I do not want to send messages to other ranks (not in comm_dist_graph).
 My question is: I create a graph with MPI_Dist_graph_create_adjacent,
which specifies a communication direction. I can do MPI_Neighbor_alltoallv
on it. That is great.  But I also want to do backward communication
(reverse the direction of the edges in graph). How can I do it without
creating a new communicator?
The background of this question is: In PETSc, we have an operation called
VecScatter, which scatters some entries of a parallel vector x to another
parallel vector y. Sometime, we want to reverse the operation, to scatter y
entries to x.


--Junchao Zhang

On Mon, Jul 23, 2018 at 9:49 AM, Jeff Hammond <jeff.science at gmail.com>
wrote:

> On Mon, Jul 23, 2018 at 7:45 AM, 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.
>>
>
> 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 <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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>>> 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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>
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/
>
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