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74
Statistical Rate Monotonic Scheduling
 In 19th IEEE RealTime Systems Symposium
, 1998
"... Statistical Rate Monotonic Scheduling (SRMS) is a generalization of the classical RMS results of Liu and Layland [LL73] for periodic tasks with highly variable execution times and statistical QoS requirements. The main tenet of SRMS is that the variability in task resource requirements could be smoo ..."
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Cited by 75 (4 self)
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Statistical Rate Monotonic Scheduling (SRMS) is a generalization of the classical RMS results of Liu and Layland [LL73] for periodic tasks with highly variable execution times and statistical QoS requirements. The main tenet of SRMS is that the variability in task resource requirements could be smoothed through aggregation to yield guaranteed QoS. This aggregation is done over time for a given task and across multiple tasks for a given period of time. Similar to RMS, SRMS has two components: a feasibility test and a scheduling algorithm. SRMS feasibility test ensures that it is possible for a given periodic task set to share a given resource without violating any of the statistical QoS constraints imposed on each task in the set. The SRMS scheduling algorithm consists of two parts: a job admission controller and ascheduler. The SRMS scheduler is a simple, preemptive, xedpriority scheduler. The SRMS job admission controller manages the QoS delivered to the various tasks through admit/reject and priority assignment decisions. In particular, it ensures the important propertyof task isolation, whereby tasks do not infringe on each other. We have evaluated SRMS against a number of alternative scheduling algorithms suggested in the literature, as well as re nements thereof. Consistently throughout our experiments, SRMS provided the best performance. In addition, to evaluate the optimality of SRMS, we havecompared it to an ine cient, yet optimal scheduler for task sets with harmonic periods.
Optimal RewardBased Scheduling of Periodic RealTime Tasks
, 1999
"... Rewardbased scheduling refers to the problem in which there is a reward associated with the execution of a task. In our framework, each realtime task comprises a mandatory and an optional part, with which a nondecreasing reward function is associated. Imprecise computation and IncreasedRewardwit ..."
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Cited by 63 (16 self)
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Rewardbased scheduling refers to the problem in which there is a reward associated with the execution of a task. In our framework, each realtime task comprises a mandatory and an optional part, with which a nondecreasing reward function is associated. Imprecise computation and IncreasedRewardwithIncreasedService models fall within the scope of this framework. In this paper, we address the rewardbased scheduling problem for periodic tasks. For linear and concave reward functions we show: (a) the existence of an optimal schedule where the optional service time of a task is constant at every instance and (b) how to efficiently compute this service time. We also prove that RMS (with harmonic periods), EDF and LLF policies are optimal when used with the optimal service times we computed, and that the problem becomes NPHard, when the reward functions are convex. Further, our solution eliminates runtime overhead, and makes possible the use of existing scheduling disciplines.
Analysis of a WindowConstrained Scheduler for RealTime and BestEffort Packet Streams
 In Proceedings of the 21st IEEE RealTime Systems Symposium
, 2000
"... This paper describes how Dynamic WindowConstrained Scheduling (DWCS) can guarantee realtime service to packets from multiple streams with different performance objectives. We show that: (1) DWCS can guarantee that no more than x packets miss their deadlines for every y consecutive packets requiring ..."
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Cited by 60 (15 self)
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This paper describes how Dynamic WindowConstrained Scheduling (DWCS) can guarantee realtime service to packets from multiple streams with different performance objectives. We show that: (1) DWCS can guarantee that no more than x packets miss their deadlines for every y consecutive packets requiring service, as long as the minimum aggregate bandwidth requirement of all realtime packet streams does not exceed the available bandwidth, (2) using DWCS, the delay of service to realtime packet streams is bounded even when the scheduler is overloaded, (3) DWCS can ensure that the delay bound of any given stream is independent of other streams, and (4) a fast response time for besteffort packet streams, in the presence of realtime packet streams, is possible. Furthermore, if a feasible schedule exists, each stream is guaranteed a minimum fraction of available bandwidth over a finite window of time. 1. Introduction Many realtime, distributed applications, such as telemedicine, virtual env...
Weakly hard realtime systems
 IEEE Transactions on Computers
, 2001
"... AbstractÐIn a hard realtime system, it is assumed that no deadline is missed, whereas, in a soft or firm realtime system, deadlines can be missed, although this usually happens in a nonpredictable way. However, most hard realtime systems could miss some deadlines provided that it happens in a kno ..."
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Cited by 44 (0 self)
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AbstractÐIn a hard realtime system, it is assumed that no deadline is missed, whereas, in a soft or firm realtime system, deadlines can be missed, although this usually happens in a nonpredictable way. However, most hard realtime systems could miss some deadlines provided that it happens in a known and predictable way. Also, adding predictability on the pattern of missed deadlines for soft and firm realtime systems is desirable, for instance, to guarantee levels of quality of service. We introduce the concept of weakly hard realtime systems to model realtime systems that can tolerate a clearly specified degree of missed deadlines. For this purpose, we define four temporal constraints based on determining a maximum number of deadlines that can be missed during a window of time �a given number of invocations). This paper provides the theoretical analysis of the properties and relationships of these constraints. It also shows the exact conditions under which a constraint is harder to satisfy than another constraint. Finally, results on fixed priority scheduling and responsetime schedulability tests for a wide range of process models are presented.
Tardiness bounds under global EDF scheduling on a multiprocessor
 In Proceedings of the 26th IEEE RealTime Systems Symposium
, 2005
"... This paper considers the scheduling of soft realtime sporadic task systems under global EDF on an identical multiprocessor. Though Pfair scheduling is theoretically optimal for hard realtime task systems on multiprocessors, it can incur significant runtime overhead. Hence, other scheduling algor ..."
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Cited by 44 (33 self)
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This paper considers the scheduling of soft realtime sporadic task systems under global EDF on an identical multiprocessor. Though Pfair scheduling is theoretically optimal for hard realtime task systems on multiprocessors, it can incur significant runtime overhead. Hence, other scheduling algorithms that are not optimal, including EDF, have continued to receive considerable attention. However, prior research on such algorithms has focussed mostly on hard realtime systems, where, to ensure that all deadlines are met, approximately 50 % of the available processing capacity will have to be sacrificed in the worst case. This may be overkill for soft realtime systems that can tolerate deadline misses by bounded amounts (i.e., bounded tardiness). In this paper, we derive tardiness bounds under preemptive and nonpreemptive global EDF on multiprocessors when the total utilization of a task system is not restricted and may equal the number of processors. Our tardiness bounds depend on pertask 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 multiprocessorbased soft realtime systems in that the latter does not offer any scope to improve system utilization even if bounded tardiness can be tolerated.
Resource reservation in dynamic realtime systems
 RealTime Systems
, 2004
"... Abstract. This paper focuses on the problem of providing ef®cient runtime support to multimedia applications in a realtime system, where different types of tasks (characterized by different criticality) can coexist.Whereas critical realtime tasks (hard tasks) are guaranteed based on worstcase ex ..."
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Cited by 37 (9 self)
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Abstract. This paper focuses on the problem of providing ef®cient runtime support to multimedia applications in a realtime system, where different types of tasks (characterized by different criticality) can coexist.Whereas critical realtime tasks (hard tasks) are guaranteed based on worstcase execution times and minimum interarrival times, multimedia tasks are served based on mean parameters.A novel bandwidth reservation mechanism (the constant bandwidth server) allows realtime tasks to execute in a dynamic environment under a temporal protection mechanism, so that each task will never exceed a prede®ned bandwidth, independently of its actual requests.The paper also discusses how the proposed server can be used for handling aperiodic tasks ef®ciently and how a statistical analysis can be applied to perform a probabilistic guarantee of soft tasks.The performance of the proposed method is compared with that of similar service mechanisms (dynamic realtime servers and proportional share schedulers) through extensive simulation experiments.
QoS Guarantee Using Probabilistic Deadlines
 IN PROCEEDINGS OF THE 11TH EUROMICRO CONFERENCE OF REALTIME SYSTEMS
, 1999
"... This paper presents a probabilistic approach to guarantee the performance of a realtime system. While traditional realtime system analysis tends to guarantee that each task instance will complete its execution before its absolute deadline (hard guarantee), our approach permits to estimate the prob ..."
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Cited by 33 (3 self)
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This paper presents a probabilistic approach to guarantee the performance of a realtime system. While traditional realtime system analysis tends to guarantee that each task instance will complete its execution before its absolute deadline (hard guarantee), our approach permits to estimate the probability that it will happen. Such a statistical guarantee is performed based on interarrival and execution times probability distributions, rather than their worst case values. The advantage of a probabilistic approach is a more efficient usage of system resources, allowing to give a certain level of deadline guarantee to task sets that the classical schedulability analysis would reject.
2003, ‘Rate Monotonic vs. EDF: Judgment Day
 In: Proc. 3rd International Conference on Embedded Software
"... Abstract. Since the first results published in 1973 by Liu and Layland on the Rate Monotonic (RM) and Earliest Deadline First (EDF) algorithms, a lot of progress has been made in the schedulability analysis of periodic task sets. Unfortunaltey, many misconceptions still exist about the properties of ..."
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Cited by 28 (3 self)
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Abstract. Since the first results published in 1973 by Liu and Layland on the Rate Monotonic (RM) and Earliest Deadline First (EDF) algorithms, a lot of progress has been made in the schedulability analysis of periodic task sets. Unfortunaltey, many misconceptions still exist about the properties of these two scheduling methods, which usually tend to favor RM more than EDF. Typical wrong statements often heard in technical conferences and even in research papers claim that RM is easier to analyze than EDF, it introduces less runtime overhead, it is more predictable in transient overload conditions, and causes less jitter in task execution. Since the above statements are either wrong, or not precise, it is time to clarify these issues in a systematic fashion, because the use of EDF allows a better exploitation of the available resources and significantly improves system’s performance. This paper compares RM against EDF under several aspects, using existing theoretical results or simple counterexamples to show that many common beliefs are either false or only restricted to specific situations. 1
Exploiting Skips In Periodic Tasks For Enhancing Aperiodic Responsiveness
 PROCEEDINGS FO REALTIME SYSTEMS SYMPOSIUM
, 1997
"... In certain realtime applications, ranging from multimedia to telecommunication systems, timing constraints can be more flexible than scheduling theory usually permits. For example, in video reception, missing a deadline is acceptable, provided that most deadlines are met. In this paper, we deal wit ..."
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Cited by 27 (3 self)
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In certain realtime applications, ranging from multimedia to telecommunication systems, timing constraints can be more flexible than scheduling theory usually permits. For example, in video reception, missing a deadline is acceptable, provided that most deadlines are met. In this paper, we deal with the problem of scheduling hybrid sets of tasks, consisting of firm periodic tasks (i.e., tasks with deadlines which can occasionally skip one instance) and soft aperiodic requests, which have to be served as soon as possible to minimize their average response time. We propose and analyze an algorithm, based on a variant of Earliest Deadline First scheduling, which exploits skips to enhance the response time of aperiodic requests. Schedulability bounds are also derived to perform offline analysis.
Dynamic WindowConstrained Scheduling for RealTime Media Streaming
 IEEE Transactions on Computers
, 2003
"... This paper describes an algorithm for scheduling packets in realtime multimedia data streams. Common to these classes of data streams are service constraints in terms of bandwidth and delay. However, it is typical for realtime multimedia streams to tolerate bounded delay variations and, in some cas ..."
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Cited by 25 (8 self)
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This paper describes an algorithm for scheduling packets in realtime multimedia data streams. Common to these classes of data streams are service constraints in terms of bandwidth and delay. However, it is typical for realtime multimedia streams to tolerate bounded delay variations and, in some cases, finite losses of packets. We have therefore developed a scheduling algorithm that assumes streams have windowconstraints on groups of consecutive packet deadlines. A windowconstraint defines the number of packet deadlines that can be missed in a window of deadlines for consecutive packets in a stream.