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
