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Subject: Re: [OMPI users] MPI_Allreduce on local machine
From: Åke Sandgren (ake.sandgren_at_[hidden])
Date: 2010-07-28 12:21:28


On Wed, 2010-07-28 at 11:48 -0400, Gus Correa wrote:
> Hi Hugo, Jeff, list
>
> Hugo: I think David Zhang's suggestion was to use
> MPI_REAL8 not MPI_REAL, instead of MPI_DOUBLE_PRECISION in your
> MPI_Allreduce call.
>
> Still, to me it looks like OpenMPI is making double precision 4-byte
> long, which shorter than I expected it be (8 bytes),
> at least when looking at the output of ompi_info --all.
>
> I always get get a size 4 for dbl prec in my x86_64 machine,
> from ompi_info --all.
> I confirmed this in six builds of OpenMPI 1.4.2: gcc+gfortran,
> gcc+pgf90, gcc+ifort, icc+ifort, pgcc+pgf90, and opencc+openf95.
> Although the output of ompi_info never says this is actually the size
> of MPI_DOUBLE_PRECISION, just of "dbl prec", which is a bit ambiguous.
>
> FWIW, I include the output below. Note that alignment for gcc+ifort
> is 1, all others are 4.
>
> Jeff: Is this correct?

This is wrong, it should be 8 and alignement should be 8 even for intel.
And i also see exactly the same thing.

-- 
Ake Sandgren, HPC2N, Umea University, S-90187 Umea, Sweden
Internet: ake_at_[hidden]   Phone: +46 90 7866134 Fax: +46 90 7866126
Mobile: +46 70 7716134 WWW: http://www.hpc2n.umu.se