Algorithms used in MPICH

1. Does MPICH use IP Multicast for MPI_Bcast?
No. In principle, MPICH could use multicast, but in practice this would be very difficult. To start with, IP multicast is unreliable; additional code to make it reliable needs to be added. In fact, there is an effort to provide a reliable multicast, built ontop of the unreliable multicast. The second problem is that not all systems allow user programs (or any program) to perform an IP multicast. In fact, that is the case for the systems that we have been developing on. Thus, we will always need the point-to-point version. There is a fairly easy way to replace any collective routine in MPI, but no-one has offered us a multicast-based MPI_Bcast yet...