Rate Monotonic Scheduling Research Articles

Imprecise Computation Model, Synchronous Periodic Real-time Task Sets and Total Weighted Error

This paper proposes two scheduling approaches, one-level and two-level scheduling, for synchronous periodic real-time task sets based on the Imprecise Computation Model. The imperative of real-time systems is a reaction on an event within a limited amount of time. Sometimes the available time and resources are not enough for the computations to complete within the deadlines, but still enough to produce approximate results. The Imprecise Computation Model is motivated by this idea, which gives the flexibility to trade off precision for timeliness. In this model a task is logically decomposed into a mandatory and optional subtask. Only the mandatory subtask is required to complete by its deadline, while the optional subtask may be left unfinished. Usually, different scheduling policies are used for the scheduling of mandatory and optional subtasks. For both proposed approaches the earliest deadline first and rate monotonic scheduling algorithms are used for the scheduling of mandatory subtasks, whereas the optional subtasks are scheduled in a way that the total weighted error is minimized. The basic idea of one-level scheduling is to extend the mandatory execution times, while in two-level scheduling the mandatory and optional subtasks are separately scheduled. The single preemptive processor model is assumed.

Non-preemptible last section assignment for reducing feedback latency in real-time control systems

A real-time control system design procedure consists of the controller design stage and the implementation stage. In the controller design stage, various digital control theories are used with assumptions, such as synchronous sampling, no sampling jitter and negligible feedback latency (latency from sensing to actuation). However, in the implementation stage, multiple control tasks are usually scheduled on a processor, which creates a finite sampling period, varying feedback latency and sampling jitter, and therefore the controller's performance is degraded. In this article, we investigate the relationship between control performance and timing parameters. In the course of this investigation, we found that the feedback latency is the dominant factor affecting control performance. In addition, we propose a rate monotonic (RM) scheduler with non-preemptible last section algorithm, which can reduce the feedback latency considerably. The proposed algorithm provides better control performance than a preemptive RM scheduler, in most cases. The effectiveness of the proposed algorithm is shown in illustrative examples.

Utilization Bound of Non-preemptive Fixed Priority Schedulers

It is known that the schedulability of a non-preemptive task set with fixed priority can be determined in pseudo-polynomial time. However, since Rate Monotonic scheduling is not optimal for non-preemptive scheduling, the applicability of existing polynomial time tests that provide sufficient schedulability conditions, such as Liu and Layland's bound, is limited. This letter proposes a new sufficient condition for non-preemptive fixed priority scheduling that can be used for any fixed priority assignment scheme. It is also shown that the proposed schedulability test has a tighter utilization bound than existing test methods.

Scheduling policy design for autonomic systems

Scheduling the execution of multiple concurrent tasks on shared resources such as CPUs and network links is essential to ensuring the reliable operation of many autonomic systems. Well-known techniques such as rate-monotonic scheduling can offer rigorous timing and preemption guarantees, but only under assumptions (i.e. a fixed set of tasks with well-known execution times and invocation rates) that do not hold in many autonomic systems. New hierarchical scheduling techniques are better suited to enforce the more flexible execution constraints and enforcement mechanisms that are required for autonomic systems, but a rigorous and efficient foundation for verifying and enforcing concurrency and timing guarantees is still needed for these approaches. This paper summarises our previous work on addressing these challenges, on Markov decision process-based scheduling policy design and on wrapping repeated structure of the scheduling state spaces involved into a more efficient model, and presents a new algorithm called expanding state policy iteration (ESPI), that allows us to compute the optimal policy for a wrapped state model.

Generalized rate monotonic schedulability bounds using relative period ratios

One of the most well-studied scheduling algorithms for real-time systems is the Rate Monotonic (RM) scheduling for periodic tasks. In this paper we derive a generalized RM schedulability bound by considering relative period ratios among tasks in a system. We show that schedulability bounds published earlier are special cases of our generalized bound. Our new bound may provide a higher value than earlier results.

Rate monotonic schedulability tests using period-dependent conditions

Feasibility and schedulability problems have received considerable attention from the real-time systems research community in recent decades. Since the publication of the Liu and Layland bound, many researchers have tried to improve the schedulability bound of the RM scheduling. The LL bound does not make any assumption on the relationship between any of the task periods. In this paper we consider the relative period ratios in a system. By reducing the difference between the smallest and the second largest virtual period values in a system, we can show that the RM schedulability bound can be improved significantly. This research has also proposed a system design methodology to improve the schedulability of real time system with a fixed system load.

Modeling and analysis for optimal PVR implementation

This paper presents a new methodology of modeling and analyzing required elements to design an HD-DTV PVR (high definition digital television personal video recorder) with efficient architecture. The bits modeling method is based on the RMS (rate monotonic scheduling) algorithm and provides a convenient way to predict the performance of real-time PVR systems and to modify its architecture with optimal resources. From the analysis, we could design PVR whose real-time performance is verified. We also developed time-shifter ASIC chip that is in charge of manipulating HD streams to support various functions like trick-mode play in the PVR

Procrastination for leakage-aware rate-monotonic scheduling on a dynamic voltage scaling processor

As the dynamic voltage scaling (DVS) technique provides system engineers the flexibility to trade-off the performance and the energy consumption, DVS has been adopted in many computing systems. However, the longer a job executes, the more energy in the leakage current the device/processor consumes for the job. To reduce the energy consumption resulting from the leakage current, a system might enter the dormant mode. This paper targets energy-efficient rate-monotonic scheduling for periodic real-time tasks on a uniprocessor DVS system with non-negligible leakage power consumption. An on-line simulated scheduling strategy and a virtually blocking time strategy are developed for procrastination scheduling to reduce energy consumption. The proposed algorithms derive a feasible schedule for real-time tasks with worst-case guarantees for any input instance. Experimental results show that our proposed algorithms could derive energy-efficient solutions.

Energy-aware task scheduling with task synchronization for embedded real-time systems

Slowdown factors determine the extent of slowdown that a computing system can experience based on functional and performance requirements. Dynamic voltage scaling (DVS) of a processor based on slowdown factors can lead to considerable energy savings. This paper addresses the problem of DVS in the presence of task synchronization. Tasks synchronize to enforce mutually exclusive access to the shared resources and can be blocked by lower priority tasks. Task slowdown factors that guarantee meeting all task deadlines are computed. Both static and dynamic priority scheduling viz. rate monotonic (RM) scheduling and earliest deadline first (EDF) scheduling, respectively, are studied

A faster exact schedulability analysis for fixed-priority scheduling

Real-time scheduling for task sets has been studied, and the corresponding schedulability analysis has been developed. Due to the considerable overheads required to precisely analyze the schedulability of a task set (referred to as exact schedulability analysis), the trade-off between precision and efficiency is widely studied. Many efficient but imprecise (i.e., sufficient but not necessary) analyses are discussed in the literature. However, how to precisely and efficiently analyze the schedulability of task sets remains an important issue. The Audsley’s Algorithm was shown to be effective in exact schedulability analysis for task sets under rate-monotonic scheduling (one of the optimal fixed-priority scheduling algorithms). This paper focuses on reducing the runtime overhead of the Audsley’s Algorithm. By properly partitioning a task set into two subsets and differently treating these two subsets during each iteration, the number of iterations required for analyzing the schedulability of the task set can be significantly reduced. The capability of the proposed algorithm was evaluated and compared to related works, which revealed up to a 55.5% saving in the runtime overhead for the Audsley’s Algorithm when the system was under a heavy load.

Dynamic voltage scaling of mixed task sets in priority-driven systems

This paper describes dynamic voltage scaling (DVS) algorithms for real-time systems with both periodic and aperiodic tasks. Although many DVS algorithms have been developed for real-time systems with periodic tasks, none of them can be used for a system with both periodic and aperiodic tasks because of the arbitrary temporal behaviors of aperiodic tasks. This paper proposes off-line and on-line DVS algorithms that are based on existing DVS algorithms. The proposed algorithms utilize the execution behaviors of scheduling servers for aperiodic tasks. Since there is a tradeoff between the energy consumption and the response time of aperiodic tasks, the proposed algorithms focus on bounding the response time degradation of aperiodic tasks although they delay the response time by stretching the task execution to get high energy savings in mixed task sets. Experimental results show that the proposed algorithms reduce the energy consumption by 48% and 35% over the non-DVS scheme under rate monotonic (RM) scheduling and earliest deadline first (EDF) scheduling, respectively.

Real-time virtual machines for avionics software migration

Generalised Rate Monotonic Scheduling (GRMS) theory has now been widely adopted in practice and supported by open standards. In recent years, Ada runtime and COTS RTOSs supporting GRMS have met the DO 178B flight control standard (http://www.ddci.com/ products_darts.shtml; http://ic.arc.nasa.gov/publications/pdf/ 2000-0213.pdf). This creates strong incentives for the avionics industry to migrate from a traditional cyclical executive based system called a federated architecture to a GRMS based system to take advantage of GRMS's flexibility, and theoretical schedulability analysis. However, the industry is reluctant to migrate due to enormous potential re-certification cost. This paper presents a novel software architecture called a Logical Federated Architecture (LFA) that greatly reduces re-certification cost due to the change of scheduling methods.

양극단 제약을 갖는 비주기, 주기 태스크와 메시지 스케줄링

The scheduling methods of the distributed real-time systems have been proposed. However, they have some weak points. They did not schedule both sporadic and periodic tasks and messages at the same time or did not consider the end-to-end constraints such as precedence relations between sporadic tasks. This means that system scheduling must guarantee the constraints of practical systems and be applicable to them. This paper proposes a new scheduling method that can be applied to more practical model of distributed real-time systems. System model consists of sporadic and periodic tasks with precedence relations and sporadic and periodic messages and has end-to-end constraints. The proposed method is based on a binary search-based period assignment algorithm, an end-to-end laxity-based priority assignment algorithm, and three kinds of schedulability analysis, node, network, and end-to-end schedulability analysis. In addition, this paper describes the application model of sporadic tasks with precedence constraints in a distributed real-time system, shows that existing scheduling methods such as Rate Monotonic scheduling are not proper to be applied to the system having sporadic tasks with precedence constraints, and proposes an end-to-end laxity-based priority assignment algorithm.

An Improved Rate Monotonic Schedulability Test Algorithm

One of the key issues in real-time theory is the schedulable analysis of a given task set with fixed priority. All the proposed schedulability test methods can be classified into two types: polynomial time methods and exact methods. Polynomial time methods use a sufficient schedulability condition, and several least upper bounds of the processor utilization under ideal assumptions have been developed. Exact methods based on the necessary and sufficient schedulability condition guarantee that the test result for every task set is correct. However, the pseudo-polynomial complexity of the exact tests is too complex to be executed online for large task sets. This paper presents a novel ISTA (improved schedulability test algorithm) for analyzing the schedulability of periodic task sets under Rate Monotonic priority assignment. A pruning theorem for the Task_i space is derived, the schedulability correlation among tasks and its effect on the schedulability test are investigated, and the related theorems are proven. Based on the above results, a new improved pseudo-polynomial schedulability test algorithm is developed, and several comparative simulation experiments among the three methods are conducted. Extensive simulations show that the average schedulability test performance as a function of number of the tasks can be significantly improved.

An Improved Rate Monotonic Schedulability Test Algorithm

An Improved Rate Monotonic Schedulability Test Algorithm

Minimum and maximum utilization bounds for multiprocessor rate monotonic scheduling

The utilization bound for real-time rate monotonic (RM) scheduling on uniprocessors is extended to multiprocessors with partitioning-based scheduling. This allows fast schedulability tests to be performed on multiprocessors and quantifies the influence of key parameters, such as the number of processors and task sizes on the schedulability of the system. The multiprocessor utilization bound is a function of the allocation algorithm, so among all the allocation algorithms there exists at least one allocation algorithm providing the minimum multiprocessor utilization bound, and one allocation algorithm providing the maximum multiprocessor utilization bound. We prove that the multiprocessor utilization bound associated with the allocation heuristic worst fit (WF) coincides with that minimum if we use Liu and Layland's bound (LLB) as the uniprocessor schedulability condition. In addition, we present a class of allocation algorithms sharing the same multiprocessor utilization bound which coincides with the aforementioned maximum using LLB. The heuristics first fit decreasing (FFD) and best fit decreasing (BFD) belong to this class. Thus, not even an optimal allocation algorithm can guarantee a higher multiprocessor utilization bound than that of FFD and BFD using LLB. Finally, the pessimism of the multiprocessor utilization bounds is estimated through extensive simulations.

A Model for Updating Real-Time Applications

Updating application software is a common occurrence for modern computing systems. Software updates stem from the need to correct coding errors or to enhance the functionality of an application. Updating an application typically requires taking the current application offline and restarting a new application. This method of updating an application is perfectly acceptable for many general purpose-computing environments. However, in real-time environments that require high availability and have stringent timing constraints, taking a process offline for updates may be unacceptable or pose unnecessary risks. Some examples of these environments include telecommunications, air traffic control, railway control and medical patient monitoring. We present a new method to dynamically update a real-time application without having to take it offline. Our new method, which we call dynamic update for real-time systems, can be used to update real-time applications using rate-monotonic scheduling, while preserving the original deadline guarantees.

Rate-monotonic scheduling on uniform multiprocessors

The rate-monotonic algorithm is arguably one of the most popular algorithms for scheduling systems of periodic real-time tasks. The rate-monotonic scheduling of systems of periodic tasks on uniform multiprocessor platforms is considered here. A simple, sufficient test is presented for determining whether a given periodic task system will be successfully scheduled by this algorithm upon a particular uniform multiprocessor platform-this test generalizes earlier results concerning rate-monotonic scheduling upon identical multiprocessor platforms.

An improved rate-monotonic admission control and its applications

Rate-monotonic scheduling (RMS) is a widely used real-time scheduling technique. This paper proposes RBound, a new admission control for RMS. RBound has two interesting properties. First, it achieves high processor utilization under certain conditions. We show how to obtain these conditions in a multiprocessor environment and propose a multiprocessor scheduling algorithm that achieves a near optimal processor utilization. Second, the framework developed for RBound remains close to the original RMS framework (that is, task dispatching is still done via a fixed-priority scheme based on the task periods). In particular, we show how RBound can be used to guarantee a timely recovery in the presence of faults and still achieve high processor utilization. We also show how RBound can be used to increase the processor utilization when aperiodic tasks are serviced by a priority exchange server or a deferrable server.

Rate-monotonic scheduling on uniform multiprocessors

Rate-monotonic scheduling on uniform multiprocessors