Hi Jeff, i would like to understand why, if i run over 512 procs or more, my code stops over mpi collective, also with little send buffer. All processors are locked into call, doing nothing. But, if i add MPI_Barrier after MPI collective, it works! I run over Infiniband net.
I know many people with this strange problem, i think there is a strange interaction between Infiniband and OpenMPI that causes it. 2009/1/23 Jeff Squyres <jsquy...@cisco.com>: > On Jan 23, 2009, at 6:32 AM, Gabriele Fatigati wrote: > >> I've noted that OpenMPI has an asynchronous behaviour in the collective >> calls. >> The processors, doesn't wait that other procs arrives in the call. > > That is correct. > >> This behaviour sometimes can cause some problems with a lot of >> processors in the jobs. > > Can you describe what exactly you mean? The MPI spec specifically allows > this behavior; OMPI made specific design choices and optimizations to > support this behavior. FWIW, I'd be pretty surprised if any optimized MPI > implementation defaults to fully synchronous collective operations. > >> Is there an OpenMPI parameter to lock all process in the collective >> call until is finished? Otherwise i have to insert many MPI_Barrier >> in my code and it is very tedious and strange.. > > As you have notes, MPI_Barrier is the *only* collective operation that MPI > guarantees to have any synchronization properties (and it's a fairly weak > guarantee at that; no process will exit the barrier until every process has > entered the barrier -- but there's no guarantee that all processes leave the > barrier at the same time). > > Why do you need your processes to exit collective operations at the same > time? > > -- > Jeff Squyres > Cisco Systems > > _______________________________________________ > users mailing list > us...@open-mpi.org > http://www.open-mpi.org/mailman/listinfo.cgi/users > > -- Ing. Gabriele Fatigati Parallel programmer CINECA Systems & Tecnologies Department Supercomputing Group Via Magnanelli 6/3, Casalecchio di Reno (BO) Italy www.cineca.it Tel: +39 051 6171722 g.fatigati [AT] cineca.it
