Bill Gropp's statement on the Forum list is clear: null handles cannot be used unless explicitly permitted. Unfortunately, there is no exception for MPI_DATATYPE_NULL when count=0. Hopefully, we will add one in MPI-4.
While your usage model is perfectly reasonable to me and something that I would do in the same position, you need to use e.g. MPI_BYTE instead to comply with the current MPI standard. As to why Open-MPI wastes CPU cycles testing for datatype validity when count=0, that is a question for someone else to answer. Implementations have no obligation enforce every letter of the MPI standard. Jeff On Wed, Jan 13, 2016 at 6:11 AM, Jim Edwards <jedwa...@ucar.edu> wrote: > It seems to me that when there is a question of interpretation of the > standard one should ask the consequences of each potential interpretation. > It just makes sense that MPI_DATATYPE_NULL should be allowed when the > count is 0, otherwise you need to insert some random datatype just to fill > the array. > > Can you make any argument in support of not allowing it (other than that's > the way you've interpreted the standard)? > > On Tue, Jan 12, 2016 at 10:44 PM, Gilles Gouaillardet < > gilles.gouaillar...@gmail.com> wrote: > >> Thanks Jeff, >> >> i found it at http://lists.mpi-forum.org/mpi-forum/2016/01/3152.php >> >> i'd like to re-iterate what i wrote earlier about example 4.23 >> MPI_DATATYPE_NULL is used as a recv type on non root tasks, >> and per the mpi 3.1 standard, recv type is "significant only at root" >> >> in the case of MPI_Gatherv, MPI_DATATYPE_NULL is *not* significant, >> but in the case of MPI_Alltoallw, it *is* significant. >> >> as far as i am concerned, and to say the least, these are two distinct >> shades of grey. >> >> >> IMHO, it would be more intuitive if the use of MPI_DATATYPE_NULL was >> allowed with a zero count, and in both MPI_Alltoallw *and* >> MPI_Sendrecv. >> >> >> i still believe George interpretation is the correct one, and Bill >> Gropp agreed with him. >> >> >> and btw, is example 4.23 correct ? >> /* fwiw, i did copy/paste it and found several missing local variable >> myrank, i, and comm >> and i'd rather have MPI_COMM_WORLD than comm */ >> >> and what if recvcount is negative on non root task ? >> should it be an error (negative int) or not (not significant value) ? >> >> Cheers, >> >> Gilles >> >> >> On Wed, Jan 13, 2016 at 2:15 PM, Jeff Hammond <jeff.scie...@gmail.com> >> wrote: >> > There's a thread about this on the MPI Forum mailing list already ;-) >> > >> > Jeff >> > >> > >> > On Tuesday, January 12, 2016, Gilles Gouaillardet <gil...@rist.or.jp> >> wrote: >> >> >> >> Jim, >> >> >> >> if i understand correctly, George point is that OpenMPI is currently >> >> correct with respect to the MPI standard : >> >> MPI_DATATYPE_NULL is *not* a predefined datatype, hence it is not >> >> (expected to be) a committed datatype, >> >> and hence it cannot be used in MPI_Alltoallw (regardless the >> corresponding >> >> count is zero). >> >> >> >> an other way to put this is mpich could/should have failed and/or you >> were >> >> lucky it worked. >> >> >> >> George and Jeff, >> >> >> >> do you feel any need to ask MPI Forum to clarify this point ? >> >> >> >> >> >> Cheers, >> >> >> >> Gilles >> >> >> >> On 1/13/2016 12:14 PM, Jim Edwards wrote: >> >> >> >> Sorry there was a mistake in that code, >> >> stypes and rtypes should be of type MPI_Datatype not integer >> >> however the result is the same. >> >> >> >> *** An error occurred in MPI_Alltoallw >> >> >> >> *** reported by process [204406785,1] >> >> >> >> *** on communicator MPI_COMM_WORLD >> >> >> >> *** MPI_ERR_TYPE: invalid datatype >> >> >> >> >> >> >> >> >> >> On Tue, Jan 12, 2016 at 7:55 PM, Jim Edwards <jedwa...@ucar.edu> >> wrote: >> >>> >> >>> Maybe the example is too simple. Here is another one which >> >>> when run on two tasks sends two integers from each task to >> >>> task 0. Task 1 receives nothing. This works with mpich and impi >> >>> but fails with openmpi. >> >>> >> >>> #include <stdio.h> >> >>> #include <mpi.h> >> >>> >> >>> my_mpi_test(int rank, int ntasks) >> >>> { >> >>> MPI_Datatype stype, rtype; >> >>> int sbuf[2]; >> >>> int rbuf[4]; >> >>> >> >>> int slen[ntasks], sdisp[ntasks], stypes[ntasks], rlen[ntasks], >> >>> rdisp[ntasks], rtypes[ntasks]; >> >>> sbuf[0]=rank; >> >>> sbuf[1]=ntasks+rank; >> >>> slen[0]=2; >> >>> slen[1]=0; >> >>> stypes[0]=MPI_INT; >> >>> stypes[1]=MPI_DATATYPE_NULL; >> >>> sdisp[0]=0; >> >>> sdisp[1]=4; >> >>> if(rank==0){ >> >>> rlen[0]=2; >> >>> rlen[1]=2; >> >>> rtypes[0]=MPI_INT; >> >>> rtypes[1]=MPI_INT; >> >>> rdisp[0]=0; >> >>> rdisp[1]=8; >> >>> >> >>> }else{ >> >>> rlen[0]=0; >> >>> rlen[1]=0; >> >>> rtypes[0]=MPI_DATATYPE_NULL; >> >>> rtypes[1]=MPI_DATATYPE_NULL; >> >>> rdisp[0]=0; >> >>> rdisp[1]=0; >> >>> } >> >>> >> >>> MPI_Alltoallw(sbuf, slen, sdisp, stypes, rbuf, rlen, rdisp, rtypes, >> >>> MPI_COMM_WORLD); >> >>> if(rank==0){ >> >>> printf("%d %d %d %d\n",rbuf[0],rbuf[1],rbuf[2],rbuf[3]); >> >>> } >> >>> >> >>> int main(int argc, char **argv) >> >>> { >> >>> int rank, ntasks; >> >>> >> >>> MPI_Init(&argc, &argv); >> >>> >> >>> MPI_Comm_rank(MPI_COMM_WORLD,&rank); >> >>> MPI_Comm_size(MPI_COMM_WORLD, &ntasks); >> >>> >> >>> printf("rank %d ntasks %d\n",rank, ntasks); >> >>> >> >>> my_mpi_test(rank,ntasks); >> >>> >> >>> >> >>> MPI_Finalize(); >> >>> } >> >>> >> >>> >> >>> >> >> >> >> >> >> >> >> -- >> >> 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/2016/01/28258.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/2016/01/28261.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/2016/01/28262.php >> > > > > -- > 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/2016/01/28266.php > -- Jeff Hammond jeff.scie...@gmail.com http://jeffhammond.github.io/
