How do the different types of point to point communication in MPI differ in terms of performance?

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The latest version of MPI includes these types of point to point (p-p) communication:

  • blocking
  • ordinary non-blocking
  • persistent non-blocking
  • partitioned non-blocking

As far as I know, historically blocking p-p communication was the first type that existed. Then different types of non-blocking p-p communication were introduced one after the other to increase performance. For example, they allow overlap of computation and communication. But are there cases where blocking p-p communication is actually faster than the non-blocking alternatives? If no, what does justify their existence? Simply backward compatibility and their simplicity of use?

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1 Answers

It is a misconception that non-blocking communication was motivated by performance: it was mostly to be able to be able to express deadlock/serialization-free communication patterns. (Indeed, actually getting overlap of communication/computation was only possible through the "Iprobe trick". Only recently with "progress threads" has it become a more systematic possibility.)

Partitioned communication is intended for multi-threaded contexts. You may call that a performance argument, or a completely new use case.

Persistent sends have indeed the potential for performance improvement, since various setups and buffer allocations can be amortized. However, I don't see much evidence that MPI implementations actually do this. https://arxiv.org/abs/1809.10778

Finally, you're missing buffered, synchronized, and ready sends. These indeed have the potential to improve performance, though again I don't see much evidence that they do.

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