Minimizing the effects of jitter in distributed hard real-time systems

In this paper we present a study of the effects caused in distributed real-time systems by the presence of jitter in the activation of tasks and messages. We show that although jitter has usually a small impact in the schedulability of single-processor systems, in distributed architectures the worst-case response times are significantly delayed. Reducing or eliminating jitter in these systems can increase the schedulability of the system up to 50% more than when jitter is permitted. The delay effects caused by jitter can be prevented by using a bandwidth-preserving scheduling algorithm such as the sporadic server. Since this kind of scheduling policy is not designed for communication networks, in this paper we describe how to adapt and implement the sporadic server algorithm for communication networks. Using the sporadic server both in the processors and networks, we can build distributed systems with up to 100% utilization of the CPUs and communication resources, while still guaranteeing that hard real-time requirements are met. ___________________ This work has been supported in part by the Comisión Interministerial de Ciencia y Tecnología of the Spanish Government, under Grant TAP94-996