Preference-oriented real-time scheduling and its application in fault-tolerant systems

In this paper, we consider a set of real-time periodic tasks where some tasks are preferably executed as soon as possible (ASAP) and others as late as possible (ALAP) while still meeting their deadlines. After introducing the idea of preference-oriented (PO) execution, we formally define the concept of PO-optimality. For fully-loaded systems (with 100% utilization), we first propose a PO-optimal scheduler, namely ASAP-Ensured Earliest Deadline (SEED), by focusing on ASAP tasks where the optimality of ALAP tasks' preference is achieved implicitly due to the harmonicity of the PO-optimal schedules for such systems. Then, for under-utilized systems (with less than 100% utilization), we show the discrepancies between different PO-optimal schedules. By extending SEED, we propose a generalized Preference-Oriented Earliest Deadline (POED) scheduler that can obtain a PO-optimal schedule for any schedulable task set. The application of the POED scheduler in a dual-processor fault-tolerant system is further illustrated. We evaluate the proposed PO-optimal schedulers through extensive simulations. The results show that, comparing to that of the well-known EDF scheduler, the scheduling overheads of SEED and POED are higher (but still manageable) due to the additional consideration of tasks' preferences. However, SEED and POED can achieve the preference-oriented execution objectives in a more successful way than EDF. The real-time scheduling theory has been studied for decades and many well-known scheduling algorithms have been proposed for various task and system models. For instance, for a set of periodic tasks running on a uniprocessor system, the rate monotonic (RM) and earliest-deadline-first (EDF) scheduling policies are shown to be optimal for static and dynamic priority based preemptive scheduling algorithms, respectively [16]. With the main objective of meeting all the timing constraints, most existing scheduling algorithms (e.g., EDF and RM) prioritize and schedule tasks based only on their timing parameters (e.g., deadlines and periods). Moreover, these algorithms usually adopt the work conservation strategy (that is, the processor will not idle if there are tasks ready for execution) and execute tasks as soon as possible (ASAP). However, there are occasions where it can be beneficial to execute tasks as late as possible (ALAP). For example, to provide better response time for soft aperiodic tasks, the earliest deadline latest (EDL) algorithm has been developed to execute periodic tasks at their latest times provided that all the deadlines will still be met [8]. By delaying the execution of all periodic tasks as much as possible , EDL has been shown …