Priority inversion and its control: An experimental investigation

To successfully engineer a large scale real-time system project, we need a disciplined approach to both the logical complexity and the timing complexity inherent in these systems. The logical complexity is managed by the software engineering methodology embodied in Ada while the timing complexity is supported by formal real-time scheduling algorithms [3,4,5,6]. From a software engineering point of view, formal scheduling algorithms translate the complex timing constraints into simple resource utilization constraints. As long as the utilization constraints of CPU, I/O channel and communication media are observed, the deadlines of periodic tasks and the response time requirements of aperiodic tasks will both be met. There is considerable freedom to modify software provided that the resource utilization remains within specified bounds. Furthermore, should there be a transient overload, the number of tasks missing their deadlines will be a function of the magnitude of the overload and the order in which the tasks miss deadlines is pre-defined [7]. From an implementation point of view, these algorithms belong to the class of static priority algorithms which can be implemented efficiently. Task priorities are functions of the timing constraints, computation requirements and relative importance of tasks. The priorities need not be computed at run-time unless task parameters are modified.