As people have said, these time values are to be expected. All they reflect is the time difference spent in reduce waiting for the slowest process to catch up to everyone else. The barrier removes that factor by forcing all processes to start from the same place.

No mystery here - just a reflection of the fact that your processes arrive at the MPI_Reduce calls at different times.

On Sep 9, 2010, at 1:14 AM, Gabriele Fatigati wrote:

More in depth,

total execution time without Barrier is about 10000 sec.

Total execution time with Barrier+Reduce is 9453, with 128 procs.

2010/9/9 Terry Frankcombe <>

Can you clarify... those timings are what is reported for the reduction
call specifically, not the total execution time?

If so, then the difference is, to a first approximation, the time you
spend sitting idly by doing absolutely nothing waiting at the barrier.


Dr. Terry Frankcombe
Research School of Chemistry, Australian National University
Ph: (+61) 0417 163 509    Skype: terry.frankcombe

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Ing. Gabriele Fatigati

Parallel programmer

CINECA Systems & Tecnologies Department

Supercomputing Group

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