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Abstract

In this 25th year anniversary paper for the IEEE Real Time Systems Symposium, we review the key results in real-time scheduling theory and the historical events that led to the establishment of the current realtime computing infrastructure. We conclude this paper by looking at the challenges ahead of us.

The Stack Resource Policy (SRP) is a resource allocation policy which permits processes with different priorities to share a single runtime stack. It is a refinement of the Priority Ceiling Protocol (PCP) of Sha, Rajkumar and Lehoczky, which strictly bounds priority inversion and permits simple schedulability tests.With or without stack sharing, the SRP offers improvements over the PCP, by: (1) unifying the treatment of stack, reader-writer, and multiunit resources, and binary semaphores; (2) applying directly to some dynamic scheduling policies, including EDF, as well as to static priority policies; (3) with EDF scheduling, supporting a stronger schedulability test; (4) reducing the maximum number of context switches for a job execution request by a factor of two. It is at least as good as the PCP in reducing maximum priority inversion.

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A polynomial-time algorithm is presented for partitioning a collection of sporadic tasks among the processors of an identical multiprocessor platform with static-priority scheduling on each individual processor. Since the partitioning problem is easily seen to be NP-hard in the strong sense, this algorithm is not optimal. A quantitative characterization of its worst-case performance is provided in terms of sufficient conditions and resource augmentation approximation bounds. The partitioning algorithm is also evaluated over randomly generated task systems.

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