[mpich-discuss] mpich runs

[Dave Goodell]

Poorly synchronized timings of extremely short computations like cpi will tend to vary wildly, since the amount of time that the computation takes is on the same order of magnitude as the spawning time for the job.  You shouldn't read too much into the timing value that cpi prints out, it's mostly just a demonstration that calling MPI_Wtime doesn't cause a crash :)

-Dave

On Oct 20, 2010, at 9:48 AM CDT, Gus Correa wrote:

> Hi Luis, Reuti, Rajeev, list
> 
> I wonder if the different wall times really indicate an error.
> 
> Cpi.c uses MPI_Wtime(), right?
> There was a recent discussion in the OpenMPI list about
> how good a measure of time this produces, particularly
> in multi-core multi-socket systems.
> See this thread:
> http://www.open-mpi.org/community/lists/users/2010/10/14456.php
> 
> My two cents,
> Gus Correa
> 
> 
> Luis Del Castillo wrote:
>> Everytime I run mpirun -np 5 or -np 8 the results for PI are the same but the wall clock time is different. -----Original Message-----
>> From: Reuti [mailto:reuti at staff.uni-marburg.de] Sent: miércoles, 20 de octubre de 2010 08:31 a.m.
>> To: mpich-discuss at mcs.anl.gov
>> Subject: Re: [mpich-discuss] mpich runs
>> Am 20.10.2010 um 00:30 schrieb Luis Del Castillo:
>>> Hi fellows my name is Luis Alejandro Del Castillo 
>> and am  having issues to figure it out why if run mpirun twice with the same configuration give me 2 different answers.
>>> wrf at cl2master-> ./mpirun -np 5 ../examples/cpi
>>> Process 0 on cl2master
>>> Process 1 on node1
>>> Process 3 on node3
>>> Process 4 on node4
>>> Process 2 on node2
>>> pi is approximately 3.1416009869231245, Error is 0.0000083333333314
>>> wall clock time = 0.007812
>>> wrf at cl2master-> ./mpirun -np 5 ../examples/cpi
>>> Process 0 on cl2master
>>> Process 3 on node3
>>> Process 1 on node1
>>> Process 2 on node2
>>> Process 4 on node4
>> Do you mean the different order in stdout? AFAIK this cannot be predicted.
>> -- Reuti
>>> pi is approximately 3.1416009869231245, Error is 0.0000083333333314
>>> wall clock time = 0.000000
>>> wrf at cl2master->
>>> I have a Quad Core Beowulf cluster with 11 nodes (1 master , 10 slaves) all the same configuration.
>>> I want to do this because I am trying to explain the performance doing some graph.
>>> Thanks
>>>  Luis Alejandro Del Castillo Riley
>>> luis.delcastillo at cathalac.org
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