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Subject: Re: [OMPI users] MPI_Reduce performance
From: Richard Treumann (treumann_at_[hidden])
Date: 2010-09-09 16:40:17


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
Dept X2ZA / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
Tele (845) 433-7846 Fax (845) 433-8363