This paper provides an in-depth analysis of the software overheads in the MPI performance-critical path and exposes mandatory performance overheads that are unavoidable based on the MPI-3.1 specification. We first present a highly optimized implementation of the MPI-3.1 standard in which the communication stack---all the way from the application to the low-level network communication API---takes only a few tens of instructions. We carefully study these instructions and analyze the root cause of the overheads based on specific requirements from the MPI standard that are unavoidable under the current MPI standard. We recommend potential changes to the MPI standard that can minimize these overheads. Our experimental results on a variety of network architectures and applications demonstrate significant benefits from our proposed changes.

This paper proposes a novel optimization technique for MPI persistent communication, that utilizes multiple RDMA engines to carry out low latency communication. Because the interconnects used in modern supercomputers have multiple RDMA engines, the multiple communication requests specified by a persistent communication invocation can be scheduled onto the RDMA engines in an optimal way and thus result in better communication performance. Such a scheduling algorithm is not only a packing problem, but also avoids interconnect resource contentions as much as possible. The scheduling algorithm proposed in this paper balances load of RDMA engines and mitigates network link contentions in case of neighbor communication patterns such like in stencil computation. The proposed scheduling algorithm is implemented in Open MPI of K computer using RDMA functions provided as Fujitsu MPI extensions. A typical 2D stencil computation of a climate simulation code is used as a benchmark program. The experimental result shows that a factor of two speedup of communication time is achieved.