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Subject: Re: [OMPI users] MPI_Reduce performance
From: Terry Frankcombe (terry_at_[hidden])
Date: 2010-09-09 03:31:17


On Thu, 2010-09-09 at 01:24 -0600, Ralph Castain wrote:
> 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.

Yes, however, it seems Gabriele is saying the total execution time
*drops* by ~500 s when the barrier is put *in*. (Is that the right way
around, Gabriele?)

That's harder to explain as a sync issue.

> 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 <terry_at_[hidden]>
> > Gabriele,
> >
> > 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.
> >
> > Ciao
> > Terry
> >
> >
> > --
> > 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
> >
> > Via Magnanelli 6/3, Casalecchio di Reno (BO) Italy
> >
> > www.cineca.it Tel: +39 051 6171722
> >
> > g.fatigati [AT] cineca.it
> >
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