Some Results of the Earliest Deadline Scheduling Algorithm
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SIGMETRICS '86/PERFORMANCE '86 Proceedings of the 1986 ACM SIGMETRICS joint international conference on Computer performance modelling, measurement and evaluation
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Journal of the ACM (JACM)
Deterministic Processor Scheduling
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This paper discusses problems associated with scheduling periodic tasks on a uniprocessor in a hard, real-time processing environment using a static-priority, preemptive-resume operating system. The scheduling problems associated with a task set containing a single periodic task which has two fixed release periods of unequal length are examined. Some real-world applications may require task release times which are periodic, but whose tasking periods are not symmetric. A scheduling algorithm for task sets with a single nonsymmetric task was developed for static-priority, preemptive-resume operating systems. The nonsymmetric scheduling algorithm is based on the rate monotonic scheduling algorithm which assigns higher task priorities to tasks with shorter release periods.The effects on processor utilization using two different priority assignment schemes are examined. The first method sorts the task priorities by the average release periods. The second method sorts the task priorities using the short nonsymmetric task period with the average period lengths for the remaining tasks. A large number of task sets were generated to characterize effects of the two priority assignment methods on task set utilization levels when used with the nonsymmetric scheduling algorithm. Characterization results for the two methods indicated that the short nonsymmetric task period priority assignment had higher breakdown utilizations than the average period priority assignment method. For task sets with a low utilization nonsymmetric task, use of the short period priority assignment method resulted in little or no loss in the overall task set breakdown utilization. Maximizing the processor utilization is desirable, provided the tasks operate in a deterministic manner and meet their deadlines. The nonsymmetric scheduling algorithm allows the system designer to calculate the feasibility of a task set containing a nonsymmetric period task.