I think I like d the most but it is not a perfect solution. With d all real8 types in a common will be badly aligned and the Open MPI internal datatype will be incorrect. So I will vote for a combo: b + d.
George. On Fri, May 30, 2014 at 4:57 AM, Gilles Gouaillardet <gilles.gouaillar...@gmail.com> wrote: > Folks, > > i recently had to solve a tricky issue that involves alignment of fortran > types. > > the attached program can be used and ran on two tasks in order to evidence > the issue. > > if gfortran is used (to build both openmpi and the test case), then the test > is successful > if ifort (Intel compiler) is used (to build both openmpi and the test case), > then the test fails. > > this was mentionned in the openmpi users list quite a while ago at > http://www.open-mpi.org/community/lists/users/2010/07/13857.php > > the root cause is gfortran considers mpi_real8 must be aligned on 8 bytes > whereas ifort considers mpi_real8 does not need to be aligned. > consequently, the derived data type ddt is built with an extent of 16 > (gfortran) or 12 (ifort) > > > in order to determine the type aligment, configure builds a simple program > with c and fortran that involves common. > the default behaviour of ifort is to : > - *not* align common > - align records (aka the real8_int fortran type) > hence the mismatch and the failure. > > the default behaviour of gfortran is to align both common and records, hence > the success. > > /* i "extracted" from configure conftest.c and conftestf.f that can be used > to build the conftest binary. conftest will store the alignment in the > conftestval file */ > > i am wondering how this should be dealt by OpenMPI. > > > here is a non exhaustive list of option : > > a) do nothing, this is not related to openmpi, and even if we do something, > application built with -noalign will break. > b) advise ifort users to configure with FCFLAGS="-align zcommons" since it > is likely this is what they want > c) advise ifort users to build their application with "-noalign" to be on > the safe side (modulo a performance penalty) > d) update OpenMPI so fortran type alignment is determined via a record > instead of a common if fortran >= 90 is used > (so far, i could not find any drawback in doing that) > e) advise ifort users to create ddt with MPI_DOUBLE instead of mpi_real8 > (because this works (!), i did not dig to find out why) > f) other ... > > any thoughts ? > > Cheers, > > Gilles > > _______________________________________________ > devel mailing list > de...@open-mpi.org > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/devel > Link to this post: > http://www.open-mpi.org/community/lists/devel/2014/05/14911.php
