Don't agree here - the only synchronization point is the completion. Ibarrier can't be completed until all have entered the barrier, but each process can leave the ibarrier() call as soon as they want to.
Rich -----Original Message----- From: devel-boun...@open-mpi.org [mailto:devel-boun...@open-mpi.org] On Behalf Of George Bosilca Sent: Friday, July 06, 2012 2:48 AM To: eugene....@oracle.com; Open MPI Developers Cc: Open MPI Developers Subject: Re: [OMPI devel] non-blocking barrier No, it is not right. With the ibarrier usage you're making below, the output should be similar to the first case (all should leave at earlist at 6.0). The ibarrier is still a synchronizing point, all processes MUST reach it before anyone is allowed to leave. However, if you move the ibarrier on proc < 2 before the sleep, the output you got become possible. George On Jul 6, 2012, at 7:53, Eugene Loh <eugene....@oracle.com> wrote: > Either there is a problem with MPI_Ibarrier or I don't understand the > semantics. > > The following example is with openmpi-1.9a1r26747. (Thanks for the fix in > 26757. I tried with that as well with same results.) I get similar results > for different OSes, compilers, bitness, etc. > > % cat ibarrier.c > #include <stdio.h> > #include <unistd.h> > #include <stdlib.h> > #include <mpi.h> > > int main(int argc, char** argv) { > int i, me; > double t0, t1, t2; > MPI_Request req; > > MPI_Init(&argc,&argv); > MPI_Comm_rank(MPI_COMM_WORLD,&me); > > MPI_Barrier(MPI_COMM_WORLD); > MPI_Barrier(MPI_COMM_WORLD); > MPI_Barrier(MPI_COMM_WORLD); > t0 = MPI_Wtime(); /* set "time zero" */ > > if ( me < 2 ) sleep(3); /* two processes delay before hitting > barrier */ > t1 = MPI_Wtime() - t0; > MPI_Barrier(MPI_COMM_WORLD); > t2 = MPI_Wtime() - t0; > printf("%d entered at %3.1lf and exited at %3.1lf\n", me, t1, t2); > > if ( me < 2 ) sleep(3); /* two processes delay before hitting > barrier */ > t1 = MPI_Wtime() - t0; > MPI_Ibarrier(MPI_COMM_WORLD, &req); > MPI_Wait(&req, MPI_STATUS_IGNORE); > t2 = MPI_Wtime() - t0; > printf("%d entered at %3.1lf and exited at %3.1lf\n", me, t1, t2); > > MPI_Finalize(); > return 0; > } > % mpirun -n 4 ./a.out > 0 entered at 3.0 and exited at 3.0 > 1 entered at 3.0 and exited at 3.0 > 2 entered at 0.0 and exited at 3.0 > 3 entered at 0.0 and exited at 3.0 > 0 entered at 6.0 and exited at 6.0 > 1 entered at 6.0 and exited at 6.0 > 2 entered at 3.0 and exited at 3.0 > 3 entered at 3.0 and exited at 3.0 > > With the first barrier, no one leaves until the last process has entered. > With the non-blocking barrier, two processes enter and leave before the two > laggards arrive at the barrier. Is that right? > _______________________________________________ > devel mailing list > de...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/devel _______________________________________________ devel mailing list de...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/devel
