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344
RealTime Mach: Towards a Predictable RealTime System
"... Distributed realtime systems play a very important role in our modern society. They are used in aircraft control, communication systems, military command and control systems, factory automation, and robotics. However, satisfying the rigid timing requirements of various realtime activities in distr ..."
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Cited by 204 (28 self)
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Distributed realtime systems play a very important role in our modern society. They are used in aircraft control, communication systems, military command and control systems, factory automation, and robotics. However, satisfying the rigid timing requirements of various realtime activities in distributed realtime systems often requires ad hoc methods to tune the system's runtime behavior The objective of RealTime Mach is to develop a realtime version of the Mach kernel which provides users with a predictable and reliable distributed realtime computing environment. In this paper
On nonpreemptive scheduling of periodic and sporadic tasks
, 1991
"... Abstract: This paper examines a fundamental problem in the theory of realtime scheduling, that of scheduling a set of periodic or sporadic tasks on a uniprocessor without preemption and without inserted idle time. We exhibit a necessary and sufficient set of conditions C for a set of periodic or sp ..."
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Cited by 165 (9 self)
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Abstract: This paper examines a fundamental problem in the theory of realtime scheduling, that of scheduling a set of periodic or sporadic tasks on a uniprocessor without preemption and without inserted idle time. We exhibit a necessary and sufficient set of conditions C for a set of periodic or sporadic tasks to be schedulable for arbitrary release times of the tasks. We then show that any set of periodic or sporadic tasks that satisfies conditions C can be scheduled with an earliest deadline first (EDF) scheduling algorithm. We also address the question of schedulability of a set of tasks with specified release times. For sets of sporadic tasks with specified release times, we show that the conditions C are again necessary and sufficient for schedulability. However, for sets of periodic tasks with specified release times, the conditions C, while sufficient, are not necessary. In fact, we show that determining whether a set of periodic tasks with specified release times is schedulable is intractable (i.e., NPhard in the strong sense). Moreover, we show that the existence of a universal algorithm for scheduling periodic tasks with specified release times would imply that P = NP. 1.
Preemptively Scheduling HardRealTime Sporadic Tasks on One Processor
 In Proceedings of the 11th RealTime Systems Symposium
, 1990
"... In this paper, we consider the preemptivescheduling of hardrealtime sporadic task systems on one processor. Wefirstgive necessary and sufficient conditions for a sporadic task system to be feasible (i.e., schedulable). The conditions cannot, in general, be tested efficiently (unless P = NP). They ..."
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Cited by 163 (32 self)
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In this paper, we consider the preemptivescheduling of hardrealtime sporadic task systems on one processor. Wefirstgive necessary and sufficient conditions for a sporadic task system to be feasible (i.e., schedulable). The conditions cannot, in general, be tested efficiently (unless P = NP). They do, however, lead to a feasibilitytestthat runs in efficient pseudopolynomial time for a very large percentage of sporadic task systems. 1 Introduction Scheduling theory as it applies to hardrealtime environments  environments where the missing of a single deadline may have disastrous consequences  seems to currently be enjoying a renaissance. Hardreal time scheduling problems may concern either fixedduration tasks or recurring tasks that must be completed within a certain time frame. The problems most studied within the recurring category involve periodically recurring tasks [LL73, LM80, LM81, LW82,Mok83, BHR90]. Aperiodically or sporadically recurring tasks have also been stud...
Scheduling RealTime Applications in an Open Environment
 in Proceedings of the 18th IEEE RealTime Systems Symposium, IEEE Computer
, 1997
"... This paper focuses on the problem of providing runtime support to realtime applications and nonrealtime applications in an open system environment. It extends the twolevel hierarchical scheduling scheme in [12] for scheduling independently developed applications. The extended scheme removes the ..."
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Cited by 162 (3 self)
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This paper focuses on the problem of providing runtime support to realtime applications and nonrealtime applications in an open system environment. It extends the twolevel hierarchical scheduling scheme in [12] for scheduling independently developed applications. The extended scheme removes the following two restrictive requirements of the scheme in [12]: (1) realtime applications that are scheduled preemptively must consist solely of periodic tasks, and (2) applications must not share global resources (i.e., resources used by more than one applications). Consequently, the extended scheme allows us to deal with a much broader range of realtime applications. 1 Introduction Recent advances in realtime systems technology have given us many good schemes for scheduling hard realtime applications. Examples are [1, 2, 5, 8]. A weakness shared by most existing schemes is that schedulability analysis must be done globally (i.e., by analyzing all applications in the system together) in...
Implications of Classical Scheduling Results For RealTime Systems
 IEEE COMPUTER
, 1995
"... Important classical scheduling theory results for realtime computing are identified. Implications of these results from the perspective of a realtime systems designer are discussed. Uniprocessor and multiprocessor results are addressed as well as important issues such as future release times, pre ..."
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Cited by 130 (1 self)
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Important classical scheduling theory results for realtime computing are identified. Implications of these results from the perspective of a realtime systems designer are discussed. Uniprocessor and multiprocessor results are addressed as well as important issues such as future release times, precedence constraints, shared resources, task value, overloads, static versus dynamic scheduling, preemption versus nonpreemption, multiprocessing anomalies, and metrics. Examples of what scheduling algorithms are used in actual applications are given.
Scheduling Aperiodic Tasks in Dynamic Priority Systems
 REALTIME SYSTEMS
, 1996
"... In this paper we present five new online algorithms for servicing soft aperiodic requests in realtime systems, where a set of hard periodic tasks is scheduled using the Earliest Deadline First (EDF) algorithm. All the proposed solutions can achieve full processor utilization and enhance aperiodic ..."
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Cited by 129 (23 self)
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In this paper we present five new online algorithms for servicing soft aperiodic requests in realtime systems, where a set of hard periodic tasks is scheduled using the Earliest Deadline First (EDF) algorithm. All the proposed solutions can achieve full processor utilization and enhance aperiodic responsiveness, still guaranteeing the execution of the periodic tasks. Operation of the algorithms, performance, schedulability analysis, and implementation complexity are discussed and compared with classical alternative solutions, such as background and polling service. Extensive simulations show that algorithms with contained runtime overhead present nearly optimal responsiveness. A valuable contribution of this work is to provide the realtime system designer with a wide range of practical solutions which allow to balance efficiency against implementation complexity.
Scheduling algorithms and operating systems support for realtime systems
 PROCEEDINGS OF THE IEEE
, 1994
"... This paper summarizes the state of the realtime field in the areas of scheduling and operating system kernels. Given the vast amount of work that has been done by both the operations research and computer science communities in the scheduling area, we discuss four paradigms underlying the schedulin ..."
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Cited by 127 (1 self)
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This paper summarizes the state of the realtime field in the areas of scheduling and operating system kernels. Given the vast amount of work that has been done by both the operations research and computer science communities in the scheduling area, we discuss four paradigms underlying the scheduling approaches and present several exemplars of each. The four paradigms are: static tabledriven scheduling, static priority preemptive scheduling, dynamic planningbased scheduling, and dynamic best efSort scheduling. In the operating system context, we argue that most of the proprietary commercial kernels as well as realtime extensions to timesharing operating system kernels do not fit the needs of predictable realtime systems. We discuss several research kernels that are currently being built to explicitly meet the needs of realtime applications.
A Multiframe Model for RealTime Tasks
 IEEE Transactions on Software Engineering
, 1996
"... The wellknown periodic task model of Liu and Layland [1] assumes a worstcase execution time bound for every task and may be too pessimistic if the worstcase execution time of a task is much longer than the average. In this paper, we give a multiframe realtime task model which allows the executio ..."
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Cited by 124 (7 self)
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The wellknown periodic task model of Liu and Layland [1] assumes a worstcase execution time bound for every task and may be too pessimistic if the worstcase execution time of a task is much longer than the average. In this paper, we give a multiframe realtime task model which allows the execution time of a task to vary from one instance to another by specifying the execution time of a task in terms of a sequence of numbers. We investigate the schedulability problem for this model for the preemptive fixed priority scheduling policy. We show that a significant improvement in the utilization bound can be established in our model. This work is supported by a grant from the Office of Naval Research under grant number N000149410582. 1 INTRODUCTION 2 1 Introduction The wellknown periodic task model by Liu and Layland(L&L) [1] assumes a worstcase execution time bound for every task. While this is a reasonable assumption for processcontroltype realtime applications, it may be o...
Mixed Pfair/ERfair Scheduling of Asynchronous Periodic Tasks
, 2001
"... Pfair Scheduling was proposed by... In this paper, we introduce a workconserving variant of Pfair scheduling called "earlyrelease" fair (ERfair) scheduling. We also present a new scheduling algorithm called PD² and show that it is optimal for scheduling any mix of earlyrelease ..."
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Cited by 107 (46 self)
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Pfair Scheduling was proposed by... In this paper, we introduce a workconserving variant of Pfair scheduling called "earlyrelease" fair (ERfair) scheduling. We also present a new scheduling algorithm called PD&sup2; and show that it is optimal for scheduling any mix of earlyrelease and nonearlyrelease asynchronous, periodic tasks. In contrast, almost all prior work on Pfair scheduling has been limited to synchronous systems. PD&sup2;is an optimization of an earlier deadlinebased algorithm of Baruah, Gehrke, and Plaxton called PD; PD&sup2; uses a simpler tiebreaking scheme than PD to disambiguate equal deadlines. We present a series of counterexamples that suggest that, in general, the PD&sup2; tiebreaking mechanism cannot be simplified. In contrast to this, we show that no tiebreaking information is needed on twoprocessor systems.
Quickrelease Fair Scheduling
 In Proceedings of the 12th Euromicro Conference on RealTime Systems
, 2003
"... In prior work on multiprocessor fairness, efficient techniques with provable properties for reallocating spare processing capacity have been elusive. In this paper, we address this shortcoming by proposing a new notion of multiprocessor fairness, called quickrelease fair (QRfair) scheduling, which ..."
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Cited by 95 (36 self)
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In prior work on multiprocessor fairness, efficient techniques with provable properties for reallocating spare processing capacity have been elusive. In this paper, we address this shortcoming by proposing a new notion of multiprocessor fairness, called quickrelease fair (QRfair) scheduling, which is a derivative of Pfair scheduling that allows efficient allocation of spare capacity. Under QRfair scheduling, each task is specified by giving both a minimum and a maximum weight (i.e., processor share). The goal is to schedule each task (as the available spare capacity changes) at a rate that is (i) at least that implied by its minimum weight and (ii) at most that implied by its maximum weight. Our contributions are fourfold. First, we present a quickrelease variant of the PD Pfair scheduling algorithm called PD . Second, we formally prove that the allocations of PD always satisfy (i) and (ii). Third, we consider the problem of defining maximum weights in a way that encourages a fair distribution of spare capacity. Fourth, we present results from extensive simulation experiments that show the efficacy of PD in allocating spare capacity.