LETT: An Execution Model for Distributed Real-Time Systems

The architecture of modern vehicles is shifting from multiple rather loosely collaborating single-core systems to fewer tightly collaborating high-performance multi-core systems. This architectural shift is intended to be capable of managing the increased computational and communication effort that is required to run advanced driver assistance and automated driving software. The most widespread real-time execution model is based on bounded worst-case execution times of tasks, which can result in weaknesses with regard to data determinism in multi-core systems. Therefore, an alternative execution model in which the execution time is abstracted by a logical time has emerged. This execution model proved its feasibility in practical use cases on multi-core systems under the assumption, that data transmission can be performed in zero time. However, this assumption is difficult to meet, especially for distributed real-time systems. In this paper, we propose an execution model in which a logical transmission time is introduced in addition to the logical execution time. The proposed execution model not only allows the prerequisite of zero read and write times to be discarded, but also the scheduling of read and write processes at a fixed point in time is no longer necessary. Consequently, we validate the proposed execution model in a case study and discuss advantages and disadvantages.