Flexible tardiness bounds for sporadic real-time task systems on multiprocessors

The earliest-deadline-first (EDF) scheduling of a sporadic real-time task system on a multiprocessor may require that the total utilization of the task system, Usum, not exceed (m + 1)/2 on m processors if every deadline needs to be met. In recent work, we considered the alleviation of this under-utilization for task systems that can tolerate deadline misses by bounded amounts (i.e., bounded tardiness). We showed that if Usum ≤ m and tasks are not pinned to processors, then the tardiness of each task is bounded under both preemptive and non-preemptive EDF. The tardiness bounds that we derived are dependent upon the utilizations and execution costs of the constituent tasks, but are independent of Usum. Furthermore, any task may incur maximum tardiness. In this paper, we address the issue of supporting tasks whose tolerance to tardiness is less than that known to be possible under EDF. We propose a new scheduling policy, called EDF-hl, that is a variant of EDF, and show that under EDF-hl, any tardiness, including zero tardiness, can be ensured for a limited number of privileged tasks, and that bounded tardiness can be guaranteed to the remaining tasks if their utilizations are restricted. EDF-hl reduces to EDF in the absence of privileged tasks. The tardiness bound that we derive is a function of Usum, in addition to individual task parameters. Hence, tardiness for all tasks can be lowered by lowering Usum. An experimental evaluation of the tardiness bounds that are possible is provided. ∗ Work supported by NSF grants CNS 0309825 and CNS 0408996. The first author was also supported by an IBM Ph.D. fellowship.

Tardiness bounds under global EDF scheduling on a multiprocessor

We consider the scheduling of a sporadic real-time task system on an identical multiprocessor. Though Pfair algorithms are theoretically optimal for such task systems, in practice, their runtime overheads can significantly reduce the amount of useful work that is accomplished. On the other hand, if all deadlines need to be met, then every known non-Pfair algorithm requires restrictions on total system utilization that can approach approximately 50% of the available processing capacity. This may be overkill for soft real-time systems, which can tolerate occasional or bounded deadline misses (i.e., bounded tardiness). In this paper we derive tardiness bounds under preemptive and non-preemptive global EDF when the total system utilization is not restricted, except that it not exceed the available processing capacity. Hence, processor utilization can be improved for soft real-time systems on multiprocessors. Our tardiness bounds depend on the total system utilization and per-task utilizations and execution costs — the lower these values, the lower the tardiness bounds. As a final remark, we note that global EDF may be superior to partitioned EDF for multiprocessor-based soft real-time systems in that the latter does not offer any scope to improve system utilization even if bounded tardiness can be tolerated

Compositional Analysis Techniques For Multiprocessor Soft Real-Time Scheduling

The design of systems in which timing constraints must be met (real-time systems) is being affected by three trends in hardware and software development. First, in the past few years, multiprocessor and multicore platforms have become standard in desktop and server systems and continue to expand in the domain of embedded systems. Second, real-time concepts are being applied in the design of general-purpose operating systems (like Linux) and attempts are being made to tailor these systems to support tasks with timing constraints. Third, in many embedded systems, it is now more economical to use a single multiprocessor instead of several uniprocessor elements; this motivates the need to share the increasing processing capacity of multiprocessor platforms among several applications supplied by different vendors and each having different timing constraints in a manner that ensures that these constraints were met. These trends suggest the need for mechanisms that enable real-time tasks to be bundled into multiple components and integrated in larger settings. There is a substantial body of prior work on the multiprocessor schedulability analysis of real-time systems modeled as periodic and sporadic task systems. Unfortunately, these standard task models can be pessimistic if long chains of dependent tasks are being analyzed. In work that introduces less pessimistic and more sophisticated workload models, only partitioned scheduling is assumed so that each task is statically assigned to some processor. This results in pessimism in the amount of needed processing resources. In this dissertation, we extend prior work on multiprocessor soft real-time scheduling and construct new analysis tools that can be used to design component-based soft real-time systems. These tools allow multiprocessor real-time systems to be designed and analyzed for which standard workload and platform models are inapplicable and for which state-of-the-art uniprocessor and multiprocessor analysis techniques give results that are too pessimistic