Consider MPI_Get_accumulate with op=MPI_NO_OP, which is used to achieve atomic Get. Obviously, one does not want to allocate and describe a source buffer that will not be touched by this. This is a case like MPI_Alltoallw where (NULL,0,MPI_DATATYPE_NULL) needs to work at participating processes.
Best, Jeff On Tue, Jan 12, 2016 at 4:46 PM, Gilles Gouaillardet < gilles.gouaillar...@gmail.com> wrote: > Thanks Jeff, > > i could not find anything in the standard that says this is an invalid > usage ... so i can only agree this is a bug. > > fwiw, example 4.23 is working fine with OpenMPI > but that is a different case : with MPI_Gather and friends, recv stuff > is irrelevant on non root task. > in the case of MPI_Alltoallw and friends, no parameter is ignored. > > fortunatly, the fix is pretty trivial, so i will make a PR from now > > Cheers, > > Gilles > > > On Wed, Jan 13, 2016 at 9:37 AM, Jeff Hammond <jeff.scie...@gmail.com> > wrote: > > Example 4.23 of MPI 3.1 (it is hardly a new example, but may have a > > different number in older versions) demonstrates the use of > > (buffer=NULL,count=0,type=MPI_DATATYPE_NULL). While examples in the MPI > > standard are not normative text, this is certainly a valid use of MPI. I > > can't find a citation where it says explicitly that this is correct, but > it > > follows logically from other text. > > > > The MPICH macro MPIR_ERRTEST_USERBUFFER that is used through the code to > > test for valid user buffers begins with "if (count > 0..." and thus does > > concern itself with the type or buffer pointer when count=0. > Furthermore, > > this macro is redundantly protected with a count>0 check when used in > > MPI_Alltoallw (and other collectives). > > > > Best, > > > > Jeff > > > > > > On Tue, Jan 12, 2016 at 4:18 PM, Gilles Gouaillardet <gil...@rist.or.jp> > > wrote: > >> > >> Hi Jim, > >> > >> can you please confirm my understanding is correct : > >> > >> - OpenMPI does *not* accept MPI_DATATYPE_NULL as an input of > MPI_Alltoallw > >> - mpich does accept MPI_DATATYPE_NULL as an input of MPI_Alltoallw *if* > >> the corresponding count *is* zero > >> - mpich does *not* accept MPI_DATATYPE_NULL as an input of MPI_Alltoallw > >> *if* the corresponding count is *not* zero > >> > >> So you are considering as a bug the fact OpenMPI does not accept > >> MPI_DATATYPE_NULL *with* a zero count. > >> > >> am i correct ? > >> > >> Cheers, > >> > >> Gilles > >> > >> > >> On 1/13/2016 8:27 AM, Jim Edwards wrote: > >> > >> Hi, > >> > >> I am using OpenMPI-1.8.3 built with gcc 4.8.3 > >> and I am using an MPI_Alltoallw call to perform > >> an all to some (or some to all) communication. > >> > >> In the case in which my task is not sending (or receiving) any data I > set > >> the > >> datatype for that send or recv buffer to MPI_DATATYPE_NULL - this > >> works fine with other mpi libraries but fails in openmpi. If I set > >> the datatype to something else say MPI_CHAR - it works fine. I think > >> that this is a bug in open-mpi - would you agree? > >> > >> > >> > >> > >> -- > >> Jim Edwards > >> > >> CESM Software Engineer > >> National Center for Atmospheric Research > >> Boulder, CO > >> > >> > >> _______________________________________________ > >> users mailing list > >> us...@open-mpi.org > >> Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > >> Link to this post: > >> http://www.open-mpi.org/community/lists/users/2016/01/28249.php > >> > >> > >> > >> _______________________________________________ > >> users mailing list > >> us...@open-mpi.org > >> Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > >> Link to this post: > >> http://www.open-mpi.org/community/lists/users/2016/01/28250.php > > > > > > > > > > -- > > Jeff Hammond > > jeff.scie...@gmail.com > > http://jeffhammond.github.io/ > > > > _______________________________________________ > > users mailing list > > us...@open-mpi.org > > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > > Link to this post: > > http://www.open-mpi.org/community/lists/users/2016/01/28251.php > _______________________________________________ > users mailing list > us...@open-mpi.org > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > Link to this post: > http://www.open-mpi.org/community/lists/users/2016/01/28252.php > -- Jeff Hammond jeff.scie...@gmail.com http://jeffhammond.github.io/
