[Mpi-comments] Post MPI 3.0 suggestion

[Stephen Booth]

This comment comes under the following:

> - Wishes for additional enhancements or new functionality
>   to be taken into account after 3.0 is published. 

I think there are some cases where persistent collective requests would
make sense. Specifically as an opportunity to allow auto-tuning of the
implementation based on the call parameters.

The most compelling justification for this is to optimise AlltoAllv
operations. Unlike the AlltoAll operation these cannot easily use tree
based optimisations for small message sizes as each process only knows
that part of the communication pattern they are directly involved with.

However if the same communication pattern is to be re-used multiple
times as a persistent request then the init call (which should itself be
collective) could take the time to auto-tune a communication schedule
best suited to the specified parameters.

		Stephen

--
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|epcc| Dr Stephen P Booth             Principal Architect       |epcc|
|epcc| s.booth at epcc.ed.ac.uk          Phone 0131 650 5746       |epcc|
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