Efficient overloading techniques for primary-backup scheduling in real-time systems

In real-time systems, tasks have deadlines to be met despite the presence of faults. Primary-Backup (PB) scheme is one of the most important schemes that has been employed for fault-tolerant scheduling of real-time tasks, wherein each task has two versions and the versions are scheduled on two different processors with time exclusion. There have been techniques proposed for improving schedulability of the PB-based scheduling, of which Backup–Backup (BB) overloading is among the most popular ones. In this technique two or more backups can share/overlap in time on a processor. In this paper, we propose two new techniques that accommodate more tasks and/or tolerate faults effectively. In the first technique, called dynamic grouping, the processors are dynamically grouped into logical groups in order to achieve efficient overloading of tasks on resources, thereby improving the schedulability and the reliability of the system. In the second technique, called PB overloading, the primary of a task can share/ overlap in time with the backup of another task on a processor. The intuition is that, for a primary (or backup), the PB-overloading can assign an earlier start time than that of the BB-overloading, thereby increasing the schedulability. We conduct schedulability and reliability analysis of the proposed techniques through simulation and analytical studies. Our studies show that dynamic grouping improves the schedulability more than static grouping, and offers graceful degradation with increasing faults. Also, PB-overloading improves the schedulability more than BB-overloading, and offers reliability comparable to that of BB-overloading. The proposed techniques are generic that they can be incorporated into many fault-tolerant non-preemptive scheduling algorithms. r 2004 Elsevier Inc. All rights reserved.