[mpich-discuss] MPI_Allreduce is slow for 7 processes or more

[David Froger]

Quoting Junchao Zhang (2015-05-04 18:28:12)
> Could you try mpich3.2b2?  I tested your code with it on my laptop. My timing
> is
> 
> np    time(s)
> 2      102.57
> 4      51.285
> 8      25.6425
> 16   12.8212

I did not tried mpich3.2b2 yet (will do tomorrow), but I've ask another
question here, just on the time to execute MPI_Barrier:
http://stackoverflow.com/questions/30039802/what-control-mpi-barrier-time-to-execute

Seems that that was why `bench_mpi.cxx` was slow.
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