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Subject: Re: [OMPI users] dual cores --> timing + /usr/bin/time
From: Raymond Wan (rwan_at_[hidden])
Date: 2008-11-11 00:57:08

Hi Fabian,

On a separate topic, but related to your post here, how did you do the
timing? [Especially to so many digits of accuracy. :-) ]

I will have to time my program and I don't think /usr/bin/time would do
it. Are the numbers it report accurate [for an MPI program]? I think
the "user time" would be inaccurate since I need to get the user time of
all the processes...but the "real time" of the main process should be ok?


Fabian Hänsel wrote:
> Be warned that at least in default config running more MPI threads than
> you have cores results in dog slow code.
> Single core machine:
> $ cat my-hosts
> localhost slots=1
> $ mpirun -np 1 -hostfile my-hosts ./sort selectionsort 1024
> 1024 0.009905000seconds
> $ mpirun -np 2 -hostfile my-hosts ./sort selectionsort 1024
> 1024 4.113605000 seconds
> (on dual core both -np 1 and -np 2 run almost equally fast (only
> slightly speedup due to poor algorithm (developed for demonstration
> purposes))