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Subject: Re: [OMPI users] MPI_Reduce performance
From: Gabriele Fatigati (g.fatigati_at_[hidden])
Date: 2010-09-09 03:34:42


Mm,I don't understand.
The experiments on my appliciation shows that an intensive use of Barrier+
Reduce is more faster than a single Reduce.

2010/9/9 Ralph Castain <rhc_at_[hidden]>

> 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 <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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-- 
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