Newsgroups: comp.parallel.mpi
From: syschem@basfigw.basf-ag.de (Katja Stolz)
Subject: MPI data types
Organization: BASF Corporation
Date: 23 Aug 1995 11:54:44 GMT
Message-ID: <41f4u4$id0@basfigw.basf-corp.com>

Hi,

I'm a novice user of MPI and I've got a problem with the MPI data types
on a SGI PowerChallange (using the latest version of mpich):
The average of a field on a three-dimensional lattice that is distributed
over the processes. The results on the sub-lattices are collected by the call

 MPI_Allreduce( &value_out, &global_value_out, 1, MPI_DOUBLE,MPI_SUM, comm_3d );
(both values are defined as double)

and the final result differs from the result of the serial version of the
program by \Delta x/x \sim 1e-14. This looks to me as an incompatibility of the
intrinsic type double  of the SGI and the MPI-type MPI_DOUBLE. (The MPI type
MPI_LONG_DOUBLE gave the same result.)
How can this be fixed? Any help would be greatly appreciated.

Regards,
Christian

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e-mail: Christian.Hoffmann@zx.basf-ag.de



