Fixed-priority Scheduling Research Articles

A Review of Hierarchical Fixed Priority Scheduling

This paper focus on two level hierarchical scheduling where several real-time applications are scheduled by the fixed priority algorithms. The application with its real-time tasks is bound to a server which can be modeled as a sporadic task with special care for the schedulability analysis. Different scheduling policies and servers can be applied for hierarchical fixed priority systems, this paper gives a closer review of schedulability analysis for applications and tasks when the global and local schedulers of a system are fixed priority.

Power-aware fixed priority scheduling for sporadic tasks in hard real-time systems

We consider the canonical sporadic task model and a generalized power model.The algorithm is scheduled by RM policy.DVS and DPM are combined to reduce energy consumption.We present a static and a d...

Schedulability Analysis of DeferrableScheduling Algorithms for MaintainingReal-Time Data Freshness

Although the deferrable scheduling algorithm for fixed priority transactions ( DS-FP) has been shown to provide a better performance compared with the More-Less (ML) method, there is still a lack of any comprehensive studies on the necessary and sufficient conditions for the schedulability of DS-FP. In this paper, we first analyze the necessary and sufficient schedulability conditions for DS-FP, and then propose a schedulability test algorithm for DS-FP by exploiting the fact that there always exists a repeating pattern in a DS-FP schedule. To resolve the limitation of fixed priority scheduling in DS-FP, we then extend the deferrable scheduling to a dynamic priority scheduling algorithm called DS-EDF by applying the earliest deadline first (EDF) policy to schedule update jobs. We also propose a schedulability test for DS-EDF and compare its performance with DS-FP and ML through extensive simulation experiments. The results show that the schedulability tests are effective. Although the schedulability of DS-EDF is lower than DS-FP and the repeating patterns in DS-EDF schedules are longer than those in DS-FP due to the use of dynamic priority scheduling, the performance of DS-EDF is better than both DS-FP and ML in terms of CPU utilization and impact on lower priority application transactions.

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

Traditional fixed-priority scheduling analysis for periodic and sporadic 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 chosen 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 underutilization of a system's resources under normal operating conditions that can arise from these assumptions, several mixed-criticality scheduling approaches have been proposed. However, to date, there have been few quantitative comparisons of system schedulability or runtime overhead for the different approaches. In this article, we present a side-by-side implementation and evaluation of the known mixed-criticality scheduling approaches, for periodic and sporadic mixed-criticality tasks on uniprocessor systems, under a mixed-criticality scheduling model that is common to all these approaches. To make a fair evaluation of mixed-criticality scheduling, we also address previously open issues and propose modifications to improve particular approaches. Our empirical evaluations demonstrate that user-space implementations of mechanisms to enforce different mixed-criticality scheduling approaches can be achieved atop Linux without kernel modification, with reasonably low (but in some cases nontrivial) overhead for mixed-criticality real-time task sets.

Bandwidth allocation for fixed-priority-scheduled compositional real-time systems

Recent research in compositional real-time systems has focused on determination of a component's real-time interface parameters. An important objective in interface-parameter determination is minimizing the bandwidth allocated to each component of the system while simultaneously guaranteeing component schedulability. With this goal in mind, in this article, we explore fixed-priority schedulability in compositional setting. First we derive an efficient exact test based on iterative convergence for sporadic task systems scheduled by fixed-priority (e.g., deadline monotonic, rate monotonic) upon an explicit-deadline periodic (EDP) resource. Then we address the time complexity of the exact test by developing a fully-polynomial-time approximation scheme (FPTAS) for allocating bandwidth to components. Our parametric algorithm takes the task system and an accuracy parameter ε > 0 as input and returns a bandwidth which is guaranteed to be at most a factor (1 + ε) times the optimal minimum bandwidth required to successfully schedule the task system. We perform thorough simulation over synthetically generated task systems to compare the performance of our proposed efficient-exact and the approximate algorithm and observe a significant decrease in runtime and a very small relative error when comparing the approximate algorithm with the exact algorithm and the sufficient algorithm.

Power-aware fixed priority scheduling for sporadic tasks in hard real-time systems

In this paper, we consider the generalized power model in which the focus is the dynamic power and the static power, and we study the problem of the canonical sporadic task scheduling based on the rate-monotonic (RM) scheme. Moreover, we combine with the dynamic voltage scaling (DVS) and dynamic power management (DPM). We present a static low power sporadic tasks scheduling algorithm (SSTLPSA), assuming that each task presents its worst-case work-load to the processor at every instance. In addition, a more energy efficient approach called a dynamic low power sporadic tasks scheduling algorithm (DSTLPSA) is proposed, based on reclaiming the dynamic slack and adjusting the speed of other tasks on-the-fly in order to reduce energy consumption while still meeting the deadlines. The experimental results show that the SSTLPSA algorithm consumes 26.55–38.67% less energy than that of the RM algorithm and the DSTLPSA algorithm reduces the energy consumption up to 18.38–30.51% over the existing DVS algorithm.

Efficient algorithms for schedulability analysis and priority assignment for fixed-priority preemptive scheduling with offsets

Fixed-priority scheduling is the most common scheduling algorithm used in industry practice. Imposing fixed task release offsets is an effective technique for improving schedulability by avoiding the critical instant when all tasks are released simultaneously. In this paper, we address the problem of schedulability analysis and priority assignment for a periodic taskset with fixed-priority preemptive scheduling, where tasks have fixed offset relationships relative to each other. For exact schedulability analysis, we present an efficient algorithm for computing busy periods, and obtaining response times of all instances of a task τi in the feasibility interval once the priority-level-pi busy periods are determined. For priority assignment, we adopt Audsley’s optimal priority assignment (OPA) algorithm, and present an efficient algorithm for incremental construction of busy periods. We also present an efficient conservative algorithm for schedulability analysis of an asynchronous taskset with much improved accuracy compared to the schedulability test without offset constraints. Performance evaluation demonstrates significant performance improvements compared to existing algorithms in terms of both computation efficiency and analysis accuracy.

Interference-aware fixed-priority schedulability analysis on multiprocessors

This paper presents new schedulability tests for preemptive global fixed-priority (FP) scheduling of sporadic tasks on identical multiprocessor platform. One of the main challenges in deriving a schedulability test for global FP scheduling is identifying the worst-case runtime behavior, i.e., the critical instant, at which the release of a job suffers the maximum interference from the jobs of its higher priority tasks. Unfortunately, the critical instant is not yet known for sporadic tasks under global FP scheduling. To overcome this limitation, pessimism is introduced during the schedulability analysis to safely approximate the worst-case. The endeavor in this paper is to reduce such pessimism by proposing three new schedulability tests for global FP scheduling. Another challenge for global FP scheduling is the problem of assigning the fixed priorities to the tasks because no efficient method to find the optimal priority ordering in such case is currently known. Each of the schedulability tests proposed in this paper can be used to determine the priority of each task based on Audsley's approach. It is shown that the proposed tests not only theoretically dominate but also empirically perform better than the state-of-the-art schedulability test for global FP scheduling of sporadic tasks.

Evaluating the Performance and Behaviour of RT-XEN

Virtualization, together with real-time support emerges to be used in an increasing amount of use cases, varying from embedded systems to enterprise computing. One of the most popular open-source virtualization software’s is Xen. Its current implementation does not qualify it to be a candidate for timecritical systems. Researchers and developers extended it and claim the efficient usage of their versions in such systems. RT-Xen is one of these versions. It is a virtualization platform based on Compositional Scheduling and uses a suite of fixed-priority schedulers. This paper evaluates the performance and scheduling behaviour of RT-Xen. The test results show that only two proposed schedulers are suitable to be used for soft real-time applications.

Efficient schedulability tests for real-time embedded systems with urgent routines

Task scheduling is one of the key mechanisms to ensure timeliness in embedded real-time systems. Such systems have often the need to execute not only application tasks but also some urgent routines (e.g. error-detection actions, consistency checkers, interrupt handlers) with minimum latency. Although fixed-priority schedulers such as Rate-Monotonic (RM) are in line with this need, they usually make a low processor utilization available to the system. Moreover, this availability usually decreases with the number of considered tasks. If dynamic-priority schedulers such as Earliest Deadline First (EDF) are applied instead, high system utilization can be guaranteed but the minimum latency for executing urgent routines may not be ensured.In this paper we describe a scheduling model according to which urgent routines are executed at the highest priority level and all other system tasks are scheduled by EDF. We show that the guaranteed processor utilization for the assumed scheduling model is at least as high as the one provided by RM for two tasks, namely \(2(\sqrt{2}-1)\). Seven polynomial time tests for checking the system timeliness are derived and proved correct. The proposed tests are compared against each other and to an exact but exponential running time test.

Limited carry-in technique for real-time multi-core scheduling

Schedulability analysis has been widely studied to provide offline timing guarantees for a set of real-time tasks. The so-called limited carry-in technique, which can be orthogonally incorporated into many different multi-core schedulability analysis methods, was originally introduced for Earliest Deadline First (EDF) scheduling to derive a tighter bound on the amount of interference of carry-in jobs at the expense of investigating a pseudo-polynomial number of intervals. This technique has been later adapted for Fixed-Priority (FP) scheduling to obtain the carry-in bound efficiently by examining only one interval, leading to a significant improvement in multi-core schedulability analysis. However, such a successful result has not yet been transferred to any other non-FP scheduling algorithms. Motivated by this, this paper presents a generic limited carry-in technique that is applicable to any work-conserving algorithms. Specifically, this paper derives a carry-in bound in an algorithm-independent manner and demonstrates how to apply the bound to existing non-FP schedulability analysis methods for better schedulability.

Analysis on schedulability of fixed-priority multiprocessor scheduling

Concerning the Fixed-Priority(FP) algorithm of multiprocessor real-time scheduling,an improved schedulability test was proposed.This paper applied Baruah's window analytical framework of Earliest Deadline First(EDF) to FP,bounded the max number of higher priority tasks doing carry-in by m-1(with m being the number of processors),and thus got a new upper bound of interference a task suffered.Then,a tighter sufficient condition to determine schedulability was derived.The simulation results show the schedulability test is more efficient by increasing the number of detected schedulable task sets.

Robust Scheduling of Task Graphs under Execution Time Uncertainty

Effective multicore computing requires to make efficient usage of the computational resources on a chip. Offline mapping and scheduling can be applied to improve the performance, but classical approaches require considerable a priori knowledge of the target application. In a practical setting, precise information is often unavailable; one can then resort to approximate time and resource usage figures, but this usually requires to make conservative assumptions. The issue is further stressed if real-time guarantees must be provided. We tackle predictable and efficient nonpreemptive scheduling of multitask applications in the presence of duration uncertainty. Hard real-time guarantees are provided with limited idle time insertion, by exploiting a hybrid offline/online technique known as Precedence Constraint Posting (PCP). Our approach does not require probability distributions to be specified, relying instead on simple and cheaper-to-obtain information (bounds, average values). The method has been tested on synthetic applications/platforms and compared with an offline optimized Fixed Priority Scheduling (FPS) approach and a pure online FIFO scheduler; the results are very promising, as the PCP schedules exhibit good stability and improved average execution time (14 percent on average, up to 30 percent versus FPS and up to 40 percent versus the FIFO scheduler).

Synchronization Analysis for Hard Real-Time Multicore Systems

Multicore processors are increasingly used in real-time embedded systems. Better utilization of hard real-time systems requires accurate scheduling and synchronization analysis. In this paper, we characterize the major synchronization penalties arising from partitioned fixed priority scheduling for hard real-time tasks on multicore platform, including transitive remote preemption, multiple remote blocking, and multiple priority inversions. Subsequently, we propose a new response time analysis by improving the approach to bound task blocking time. The key idea of this approach is to classify the total blocking time into (i) direct blocking, including local and remote blocking, and transitive remote preemption; and (ii) multiple local interference which is incurred by multiple priority inversion. Simulation results indicate that the proposed approach produces less pessimistic results in task blocking time, and better schedulability performance.

Overload provisioning in mixed-criticality cyber-physical systems

Cyber-physical systems are an emerging class of applications that require tightly coupled interaction between the computational and physical worlds. These systems are typically realized using sensor/actuator interfaces connected with processing backbones. Safety is a primary concern in cyber-physical systems since the actuators directly influence the physical world. However, unexpected or unusual conditions in the physical world can manifest themselves as increased workload demands being offered to the computational infrastructure of a cyber-physical system. Guaranteeing system safety under overload conditions is therefore a prime concern in developing and deploying cyber-physical systems. In this work, we study this problem in the context of a radar surveillance system, where tasks have different levels of criticality or influence on system safety . In the face of overloads, we observe that the desirable property in such systems is that the more critical tasks continue to meet their timing requirements. We capture this mixed-criticality overload requirement using a formal overload-tolerance metric called ductility . Using this overload-tolerance metric, we first develop our solution in the context of uniprocessor systems, where we show that Zero-Slack scheduling (ZS) algorithms can be used to improve the overload behavior in mixed-criticality cyber-physical systems compared to existing fixed-priority scheduling algorithms like Rate-Monotonic Scheduling (RMS) and Criticality-As-Priority-Assignment (CAPA). Leveraging these results, we then develop a criticality-aware task allocation algorithm called Compress-on-Overload Packing (COP) for dealing with multiprocessor cyber-physical systems. Evaluation results show that COP achieves up to five times better ductility than traditional load balancing bin-packing algorithms like Worst-Fit Decreasing (WFD). Finally, we apply ZS and COP to the radar surveillance system to demonstrate the resulting improvement in system overload behavior. Our implementation of the Zero-Slack scheduler is available as a part of the Linux/RK project, which provides resource kernel extensions for Linux.

Customization of a Real-Time Operating System Scheduler with Aspect-Oriented Programming

Tasks of an application program of an embedded system are managed by the scheduler of a real-time operating system (RTOS). Most RTOSs adopt just fixed priority scheduling, which is not optimal in all cases. Some applications require earliest deadline first (EDF) scheduling, which is an optimal scheduling algorithm. In order to develop an efficient real-time embedded system, the scheduling algorithm of the RTOS should be selectable. The paper presents a method to customize the scheduler using aspectoriented programming. We define aspects to replace the fixed priority scheduling mechanism of an OSEK OS with an EDF scheduling mechanism. By using the aspects, we can customize the scheduler without modifying the original source code. We have applied the aspects to an OSEK OS and get a customized operating system with EDF scheduling. The evaluation results show that the overhead of aspect-oriented programming is small enough. Keywords—aspect-oriented programming, embedded system, operating system, real-time system

The OMLP family of optimal multiprocessor real-time locking protocols

This paper presents the first suspension-based multiprocessor real-time locking protocols with asymptotically optimal blocking bounds (under certain analysis assumptions). These protocols can be applied under any global, clustered, or partitioned job-level fixed-priority scheduler and support mutual exclusion, reader-writer exclusion, and k-exclusion constraints. Notably, the reader-writer and k-exclusion protocols are the first analytically-sound suspension-based multiprocessor real-time locking protocols of their kind. To formalize a notion of “optimal blocking,” precise definitions of what constitutes “blocking” in a multiprocessor real-time system are given and a simple complexity metric for real-time locking protocols, called maximum priority-inversion blocking (pi-blocking), is introduced. It is shown that, in a system with m processors, Ω(m) maximum pi-blocking is unavoidable. This bound is shown to be asymptotically tight with the introduction of the O(m) multiprocessor locking protocol (OMLP) family presented herein, which includes protocols that ensure an upper bound on maximum pi-blocking that is approximately within a factor of two of the lower bound. In addition to the coarse-grained asymptotic bounds, detailed blocking bounds suitable for schedulability analysis are derived using holistic blocking analysis. Based on the detailed bounds, the proposed locking protocols are compared with each other and with previously-proposed protocols in an empirical schedulability study involving more than one billion task sets. In this study, the OMLP was found to perform better than two variants of the classic (but non-optimal) multiprocessor priority-ceiling protocol (MPCP).

An experimental comparison of different real-time schedulers on multicore systems

In this work, an experimental comparison among the Rate Monotonic (RM) and Earliest Deadline First (EDF) multiprocessor real-time schedulers is performed, with a focus on soft real-time systems. We generated random workloads of synthetic periodic task sets and executed them on a big multi-core machine, using Linux as Operating System, gathering an extensive amount of data related to their exhibited performance under various real-time scheduling strategies. The comparison involves the fixed-priority scheduler for multiprocessors as available in the Linux kernel (with priorities set so as to achieve RM), and on our own implementation of EDF, both configured in global, partitioned and clustered mode. The impact of the various scheduling strategies on the performance of the applications, as well as the generated scheduling overheads, are compared presenting an extensive set of experimental results. These provide a comprehensive view of the performance achievable by the different schedulers under various workload conditions.

On-line schedulability tests for adaptive reservations in fixed priority scheduling

Adaptive reservation is a real-time scheduling technique in which each application is associated a fraction of the computational resource (a reservation) that can be dynamically adapted to the varying requirements of the application by using appropriate feedback control algorithms. An adaptive reservation is typically implemented by using an aperiodic server (e.g. sporadic server) algorithm with fixed period and variable budget. When the feedback law demands an increase of the reservation budget, the system must run a schedulability test to check if there is enough spare bandwidth to accommodate such increase. The schedulability test must be very fast, as it may be performed at each budget update, i.e. potentially at each instance of a task; yet, it must be as efficient as possible, to maximize resource usage. In this paper, we tackle the problem of performing an efficient on-line schedulability test for adaptive resource reservations in fixed priority schedulers. In the literature, a number of algorithms have been proposed for on-line admission control in fixed priority systems. We describe four of these tests, with increasing complexity and performance. In addition, we propose a novel on-line test, called Spare-Pot algorithm, which has been specifically designed for the problem at hand, and which shows a good cost/performance ratio compared to the other tests.

FPSL, FPCL and FPZL schedulability analysis

This paper presents the Fixed Priority until Static Laxity (FPSL), Fixed Priority until Critical Laxity (FPCL) and Fixed Priority until Zero Laxity (FPZL) scheduling algorithms for multiprocessor real-time systems. FPZL is similar to global fixed priority pre-emptive scheduling; however, whenever a task reaches a state of zero laxity it is given the highest priority. FPSL and FPCL are variants of FPZL that introduce no additional scheduling points beyond those present with fixed priority scheduling. FPSL, FPCL and FPZL are minimally dynamic algorithms, in that the priority of a job can change at most once during its execution, bounding the number of pre-emptions. Polynomial time and pseudo-polynomial time sufficient schedulability tests are derived for these algorithms. The tests are then improved by computing upper bounds on the amount of execution that each task can perform at the highest priority. An empirical evaluation shows that FPSL, FPCL, and FPZL are highly effective, with a significantly larger number of tasksets deemed schedulable by the tests derived in this paper, than by state-of-the-art schedulability tests for EDZL scheduling.