Ashley

Can you provide an example of a situation in which these semantically redundant barriers help?

I may be missing something but my statement for the text book would be

"If adding a barrier to your MPI program makes it run faster, there is almost certainly a flaw in it that is better solved another way."

The only exception I can think of is some sort of one direction data dependancy with messages small enough to go eagerly.  A program that calls MPI_Reduce with a small message and the same root every iteration and  calls no other collective would be an example.

In that case, fast tasks at leaf positions would run free and a slow task near the root could pile up early arrivals and end up with some additional slowing. Unless it was driven into paging I cannot imagine the slowdown would be significant though.

 Even that should not be a problem for an MPI implementation that backs off on eager send before it floods early arrival buffers.


Dick Treumann  -  MPI Team          
IBM Systems & Technology Group
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