Analytical Characterization of End-to-End Communication Delays With Logical Execution Time

Modern automotive embedded systems are composed of multiple real-time tasks communicating by means of shared variables. The effect of an initial event is typically propagated to an actuation signal through sequences of tasks writing/reading shared variables, creating an effect chain (EC). The responsiveness, performance and stability of the control algorithms of an automotive application typically depend on the propagation delays of selected ECs. Indeed, task jitter can have a negative impact on the system potentially leading to instability. The logical execution time (LET) model has been recently adopted by the automotive industry as a way of reducing jitter and improving the determinism of the system. In this paper, we provide a formal analysis of the LET model for real-time systems composed of periodic tasks with harmonic and nonharmonic periods, analytically characterizing the control performance of LET ECs. We also show that by introducing tasks offsets, the real-time performance of nonharmonic tasks may improve, getting closer to the constant end-to-end latency experienced in the harmonic case. Further, we present a heuristic algorithm to obtain a set of offsets that might reduce end-to-end latencies, improving LET communication determinism. Finally, we apply this technique to an industrial case study consisting of an automotive engine control system.