A Scheduling Strategy for a Distributed Hard Real-Time Programming Environment

Abstract

Abstract In this paper we discuss the integrated scheduling strategy adopted to try to met the timing requirements of distributed programs written in STER. Programs in STER include periodic modules (or tasks) with precedence relations, aperiodic modules and modules without timing requirements (non real-time modules). Modules can share remote resources using servers. A client module can access a server using single service requests or multiservice accesses. In a multiservice access a server is allocated in exclusive mode to the client for a given sequence of service requests. The scheduling strategy discussed in this paper consists of three levels: off-line scheduling of periodic modules and on-line scheduling of aperiodic and non real-time modules. To off-line search for a feasible schedule, a known heuristic algorithm was extended to support the concepts of module, server and multiservice accesses to servers. To increase the schedulability of aperiodic modules, the off-line scheduling algorithm implements some simple schemes to balance the load between the sites of a distributed system and, instead of rigid start times, defines scheduling windows for periodic tasks.