Non-preemptive Regions Research Articles

The Predictable Execution Model in Practice

Adoption of multi- and many-core processors in real-time systems has so far been slowed down, if not totally barred, due do the difficulty in providing analytical real-time guarantees on worst-case execution times. The Predictable Execution Model (PREM) has been proposed to solve this problem, but its practical support requires significant code refactoring, a task better suited for a compilation tool chain than human programmers. Implementing a PREM compiler presents significant challenges to conform to PREM requirements, such as guaranteed upper bounds on memory footprint and the generation of efficient schedulable non-preemptive regions. This article presents a comprehensive description on how a PREM compiler can be implemented, based on several years of experience from the community. We provide accumulated insights on how to best balance conformance to real-time requirements and performance and present novel techniques that extend the applicability from simple benchmark suites to real-world applications. We show that code transformed by the PREM compiler enables timing predictable execution on modern commercial off-the-shelf hardware, providing novel insights on how PREM can protect 99.4% of memory accesses on random replacement policy caches at only 16% performance loss on benchmarks from the PolyBench benchmark suite. Finally, we show that the requirements imposed on the programming model are well-aligned with current coding guidelines for timing critical software, promoting easy adoption.

Response Time Analysis for Tasks with Fixed Preemption Points under Global Scheduling

As an effective method for detecting the schedulability of real-time tasks on multiprocessor platforms, Response time analysis (RTA) has been deeply researched in recent decades. Most of the existing RTA methods are designed for tasks that can be preempted at any time. However, in some real-time systems, a task may have some fixed preemption points (FPPs) that divide its execution into a series of non-preemptive regions (NPRs). In such environments, the task can only be preempted at its FPPs, which makes existing RTA methods for arbitrary preemption tasks not applicable. In this article, we study the schedulability analysis on tasks with FPPs under both global fixed-priority (G-FP) scheduling and global earliest deadline first (G-EDF) scheduling. First, based on the idea of limiting the time interval between two consecutive executions of an NPR, a novel RTA method for tasks with FPPs under G-FP scheduling is proposed. Second, we propose an effective RTA method for tasks with FPPs under G-EDF scheduling. Finally, extensive simulations are conducted and the results validate the effectiveness of the proposed methods.

Using non-preemptive regions and path modification to improve schedulability of real-time traffic over priority-based NoCs

Network-on-Chip (NoC) is a preferred communication medium for massively parallel platforms. Fixed-priority based scheduling using virtual-channels is one of the promising solutions to support real-time traffic in on-chip networks. Most of the existing works regarding priority-based NoCs use a flit-level preemptive scheduling. Under such a mechanism, preemptions can only happen between the transmissions of successive flits but not during the transmission of a single flit. In this paper, we present a modified framework where the non-preemptive region of each NoC packet increases from a single flit. Using the proposed approach, the response times of certain traffic flows can be reduced, which can thus improve the schedulability of the whole network. As a result, the utilization of NoCs can be improved by admitting more real-time traffic. Schedulability tests regarding the proposed framework are presented along with the proof of the correctness. Additionally, we also propose a path modification approach on top of the non-preemptive region based method to further improve schedulability. A number of experiments have been performed to evaluate the proposed solutions, where we can observe significant improvement on schedulability compared to the original flit-level preemptive NoCs.

Global and Partitioned Multiprocessor Fixed Priority Scheduling with Deferred Preemption

This article introduces schedulability analysis for Global Fixed Priority Scheduling with Deferred Preemption (gFPDS) for homogeneous multiprocessor systems. gFPDS is a superset of Global Fixed Priority Preemptive Scheduling (gFPPS) and Global Fixed Priority Nonpreemptive Scheduling (gFPNS). We show how schedulability can be improved using gFPDS via appropriate choice of priority assignment and final nonpreemptive region lengths, and provide algorithms that optimize schedulability in this way. Via an experimental evaluation we compare the performance of multiprocessor scheduling using global approaches: gFPDS, gFPPS, and gFPNS, and also partitioned approaches employing FPDS, FPPS, and FPNS on each processor.

Feasibility analysis under fixed priority scheduling with limited preemptions

Preemptive scheduling often generates a significant runtime overhead that may increase task worst-case execution times up to 40%, with respect to a fully non-preemptive execution. In small embedded systems, such an extra cost results in longer and more variable response times that can significantly affect the overall energy consumption, as well as the system predictability. Limiting preemptions is often possible without jeopardizing schedulability. Although several authors addressed schedulability analysis under different forms of limited preemptive scheduling, current results exhibit two major deficiencies: (i) The maximum lengths of the non-preemptive regions for each task are still unknown under fixed priorities; (i) The exact response time analysis for tasks with fixed preemption points is too complex. This paper presents the schedulability analysis of real-time tasks with non-preemptive regions, under fixed priority assignments. In particular, two different preemption models are considered: the floating and the fixed preemption point model. Under each model, the feasibility analysis is addressed by deriving simple and effective schedulability tests, as well as an algorithm for computing the maximum length of the non-preemptive regions for each task. Finally, simulation experiments are presented to compare the two models in terms of schedulability.

Generalized tardiness bounds for global multiprocessor scheduling

We consider the issue of deadline tardiness under global multiprocessor scheduling algorithms. We present a general tardiness-bound derivation that is applicable to a wide variety of such algorithms (including some whose tardiness behavior has not been analyzed before). Our derivation is very general: job priorities may change rather arbitrarily at runtime, capacity restrictions may exist on certain processors, and, under certain conditions, non-preemptive regions are allowed. Our results show that, with the exception of static-priority algorithms, most global algorithms considered previously have bounded tardiness. In addition, our results provide a simple means for checking whether tardiness is bounded under newly-developed algorithms.