Implementation and evaluation of mixed-criticality scheduling approaches for periodic tasks

Traditional fixed-priority scheduling analysis for periodic task sets is based on the assumption that all tasks are equally critical to the correct operation of the system. Therefore, every task has to be schedulable under the scheduling policy, and estimates of tasks' worst case execution times must be conservative in case a task runs longer than is usual. To address the significant under-utilization of a system's resources under normal operating conditions that can arise from these assumptions, three main approaches have been proposed: priority assignment, period transformation, and zero-slack scheduling. However, to date there has been no quantitative comparison of system schedulability or run-time overhead for the different approaches. In this paper, we present what is to our knowledge the first side-by-side evaluation of those approaches, for periodic mixed-criticality tasks on uniprocessor systems, under a mixed-criticality scheduling model that is common to all three approaches. To make a fair evaluation of zero-slack scheduling, we also address two previously open issues: how to accommodate execution of a task after its deadline, and how to account for previously unidentified forms of interference between mixed-criticality tasks. Our simulations show that while priority assignment and period transformation are most likely to be able to schedule a randomly selected task set, a small fraction of the task sets are schedulable only under the zero-slack approach. Our empirical evaluation demonstrates that user-space implementations of mechanisms to enforce period transformation and zero-slack scheduling can be achieved on Linux without kernel modification, with suitably low overhead for mixed-criticality real-time task sets.