In a configuration with multiple host ports on the same fabric, what connection pattern does Open MPI use?
When multiple active ports exist on the same physical fabric between multiple hosts in an MPI job, Open MPI will attempt to use them all by default. Open MPI makes several assumptions regarding active ports when establishing connections between two hosts. Active ports that have the same subnet ID are assumed to be connected to the same physical fabric — that is to say that communication is possible between these ports. Active ports with different subnet IDs are assumed to be connected to different physical fabric — no communication is possible between them. It is therefore very important that if active ports on the same host are on physically separate fabrics, they must have different subnet IDs. Otherwise Open MPI may attempt to establish communication between active ports on different physical fabrics. The subnet manager allows subnet prefixes to be assigned by the administrator, which should be done when multiple fabrics are in use. The following is a brief description of how conn
