Prevention of task overruns in real-time non-preemptive multiprogramming systems

Abstract

Real-time multiprogramming systems, in which a hardware processor is dynamically assigned to run multiple software processes each designed to control an important device (user), are considered. Each software process executes a task in response to a service request repeatedly coming from the corresponding user. Each service task is associated with a strict deadline, and thus the design problem that we are concerned with is to ensure that the service tasks requested can always be executed within the associated deadlines, i.e., no task overrun occurs. This problem was studied by several investigators for the cases where preemptive scheduling strategies are used. In contrast, very few studies have been conducted for cases of non-preemptive scheduling. In this paper we show that a non-preemptive strategy, called relative urgency non-preemptive (RUNP) strategy, is optimal in the sense that if a system runs without a task overrun under any non-preemptive strategy, it will also run without a task overrun under the RUNP strategy. Then an efficient procedure used at the design time for detecting the possibility of a task overrun in a system using the RUNP strategy is presented. The procedure is useful in designing overrun-free real-time multiprogramming systems that yield high processor utilizations. Some special types of systems using the RUNP strategy for which even simpler detection procedures are available are also discussed.