Shaun Jackman wrote:
I've just read in the Open MPI documentation [1]
That's the MPI spec, actually.
that collective operations, such as MPI_Allreduce, may synchronize, but do not necessarily synchronize. My algorithm requires a collective operation and synchronization; is there a better (more efficient?) method than simply calling MPI_Allreduce followed by MPI_Barrier?
MPI_Allreduce is a case that actually "requires" synchronization in that no participating process may exit before all processes have entered. So, there should be no need to add additional synchronization. A special case might be an MPI_Allreduce of a 0-length message, in which case I suppose an MPI implementation could simple "do nothing", and the synchronization side-effect would be lost.
The MPI spec is mainly talking about a "typical" collective where one could imagine a process exiting before some processes have entered. E.g., in a broadcast or scatter, the root could exit before any other process has entered the operation. In a reduce or gather, the root could enter after all other processes have exited. For all-to-all, allreduce, or allgather, however, no process can exit before all processes have entered, which is the synchronization condition effected by a barrier. (Again, null message lengths can change things.)
[1] http://www.mpi-forum.org/docs/mpi21-report-bw/node85.htm _______________________________________________ users mailing list us...@open-mpi.org http://www.open-mpi.org/mailman/listinfo.cgi/users
