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Subject: Re: [OMPI users] MPI_Reduce performance
From: Gabriele Fatigati (g.fatigati_at_[hidden])
Date: 2010-09-09 03:36:28


Yes Terry,

thats' right.

2010/9/9 Terry Frankcombe <terry_at_[hidden]>

> 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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> >
> >
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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