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
