Analysing Real-Time Behaviour of Collective Communication Patterns in MPI

Worst-case execution time (WCET) analysis is crucial for designing real-time systems. While the WCET of tasks in a single core system can be upper bounded in isolation, the tasks in a manycore system are subject to shared memory interferences which impose high overestimation of the WCET bounds. However, manycore-based massively parallel applications will enter the area of real-time systems in the years ahead. Explicit message-passing and a clear separation of computation and communication facilitates WCET analysis for those programs. Thereby, the separation is especially ensured if applying collective communication. We propose a process of analysing state-of-the-art communication patterns with respect to worst case timing. As MPI is the standard for performing collective communication, we use it to show how to evaluate the timing behaviour in detail. We compare different communication patterns and show the tremendous impact of choosing an appropriate one.