Towards leveraging collective performance with the support of MPI 4.0 features in MPC

Persistent collective communications and communicator splitting according to the underlying hardware topology have recently been voted in the MPI standard. Persistent semantics contains an initialization phase called only once for a specific collective operation, with subsequent recurring invocations. This opens the door to many optimizations requiring heavy setup costs to improve collective performances. Communicator topological splitting offers a standard way to design topological algorithm through the use of sub-communicators mapped to hardware hierarchical levels. Setting these communicators might be too costly to be efficient on a single collective call. However, the persistent semantics allow to create these communicators once at initialization, and use them repeatedly in the multiple collective invocations to have an efficient algorithm.