[mpiwg-coll] [mpiwg-hw-topology] Discussing Semantics and Synchronization Requirements for Neighbourhood Collectives

[Anthony Skjellum]

FYI to Christoph and Julien, I think that alltoallv/w operations with zero
counts are deserving special consideration too … why would we require a
barrier ? ; allowing a barrier is acceptable …  with neighbors, there is
little conceptual difference from zero counts for some sources and sinks,
so we must be consistent.

The term synchronizing collective means
— must
Or
— may
?? That is confusing.

Clearly barrier must… what about persistent allreduce implemented via
1-sided inside … is there for sure a barrier effect ?

I am worried about this detail to define partitioned collectives too.

Tony

Anthony Skjellum, PhD
skjellum at gmail.com
Cell: +1-205-807-4968




On Tue, Mar 18, 2025 at 9:15 AM Christoph Niethammer via mpiwg-hw-topology <
mpiwg-hw-topology at lists.mpi-forum.org> wrote:

> Hello everyone, During the preparation for the FRM, a question was raised
> regarding whether and how the semantics of collectives apply to
> neighborhood collectives. [1] This question stemmed from remark 18) in the
> summary table in Appendix A. 2,
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> Hello everyone,
>
> During the preparation for the FRM, a question was raised regarding
> whether and how the semantics of collectives apply to neighborhood collectives. [1]
> This question stemmed from remark 18) in the summary table in Appendix A.2,
> which was used to apply the semantics of collectives to neighbourhood
> collectives at this place.
>
> The discussions expanded during the March meeting to the question of whether remark 18,
> stating that "all-to-all collectives *must* synchronize (with some restrictions)",
> is correct. [2]
>
> I would like to invite you all to discuss these two issues in more detail during a
> Topologies WG meeting on March 27th at 15:00, as part of our regular WG slot.
>
> Best regards
> Christoph Niethammer
>
> [1] https://urldefense.us/v3/__https://github.com/mpi-forum/mpi-issues/issues/863__;!!G_uCfscf7eWS!edo6G3QE7JbjtZTTusbpg5yIgnpGmlkZf6PJXEhK0BIFo_p25xzgJ-iVjVZhEuL7z3exLA62R0MjXhpEVDjC5QAigTtN4tiyDa1MnE8$
> [2] https://urldefense.us/v3/__https://github.com/mpi-forum/mpi-issues/issues/971__;!!G_uCfscf7eWS!edo6G3QE7JbjtZTTusbpg5yIgnpGmlkZf6PJXEhK0BIFo_p25xzgJ-iVjVZhEuL7z3exLA62R0MjXhpEVDjC5QAigTtN4tiyWCbeaNg$
>
> --
>
> Dr.-Ing. Christoph Niethammer
> High Performance Computing Center Stuttgart (HLRS)Nobelstrasse 19
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>
> Tel: ++49(0)711-685-87203
> email: christoph.niethammer at hlrs.dehttps://urldefense.us/v3/__https://www.hlrs.de/people/christoph-niethammer__;!!G_uCfscf7eWS!edo6G3QE7JbjtZTTusbpg5yIgnpGmlkZf6PJXEhK0BIFo_p25xzgJ-iVjVZhEuL7z3exLA62R0MjXhpEVDjC5QAigTtN4tiyVz0SIxo$
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