[Mpi-comments] One comment on MPI-3.0 Draft 2, August 2012

[William Gropp]

This is a good point.  Using the Testsome or Waitsome interface permits an application to respond fairly without burdening the implementation with any particular fairness definition.  Not every code can be conveniently written this way, but it addresses many needs.

As has been noted, specifying the exact meaning of fairness (e.g., round robin) may have serious performance impact, as an implementation may make fair progress but in a different, possibly non-deterministic, order.  Even if "fairness" is provided, it should be defined in terms of the essential properties, not a particular implementation that happens to have those properties.

Bill

William Gropp
Director, Parallel Computing Institute
Deputy Director for Research
Institute for Advanced Computing Applications and Technologies
Paul and Cynthia Saylor Professor of Computer Science
University of Illinois Urbana-Champaign



On Aug 15, 2012, at 8:54 AM, Jim Dinan wrote:

> Hi Sebastien,
> 
> Something to consider in addition to probe is that fairness semantics are also relevant to test/wait operations that take arrays of requests.  It may make the most sense to attach a fairness attribute to communication contexts (communicators and windows) to provide more flexibility.  This could already be done by an MPI implementation through an info key on a communicator/window, however implementations need not support all info keys and fairness could be a requirement for applications that rely on it.
> 
> Cheers,
> ~Jim.
> 
> On 8/15/12 10:19 AM, Sébastien Boisvert wrote:
>> Maybe there could be something like MPI_Init_fairness which could either
>> be MPI_FAIRNESS_NO (the current behavior) or MPI_FAIRNESS_YES (which
>> would do round-robin reception).
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