Scheduling Aperiodic and Sporadic Tasks in Hard Real-Time Systems

The stringent timing constraints as well as the functional correctness are essential requirements of hard real-time systems. In such systems, scheduling plays a very important role in satisfying these constraints. The priority based scheduling schemes have been used commonly due to the simplicity of the scheduling algorithm. However, in the presence of task interdependencies and complex timing constraints, such scheduling schemes may not be appropriate due to the lack of an efficient mechanism to schedule them and to carry out the schedulability analysis. In contrast, the time based scheduling scheme may be used to schedule a set of tasks with greater degree of schedulability achieved at a cost of higher complexity of off-line scheduling. One of the drawbacks of currently available scheduling schemes, however, is known to be their inflexibility in dynamic environments where dynamic processes exist, such as aperiodic and sporadic processes. We develop and analyze scheduling schemes which efficiently provide the flexibility required in real-time systems for scheduling processes arriving dynamically. This enables static hard periodic processes and dynamic processes(aperiodic or sporadic) to be jointly scheduled. (Also cross-referenced as UMIACS-TR-97-44

Designing Temporal Controls

Traditional control systems have been designed to exercise control at regularly spaced time instants. When a discrete version of the system dynamics is used, a constant sampling interval is assumed and a new control value is calculated and exercised at each time instant. In this paper we formulate a new control scheme, {\it temporal control}, in which we not only calculate the control value but also decide the time instants when the new values are to be used. Taking a discrete, linear, time-invariant system, and a cost function which reflects a cost for computation of the control values, as an example, we show the feasibility of using this scheme. We formulate the temporal control scheme as a feedback scheme and, through a numerical example, demonstrate the significant reduction in cost through the use of temporal control. (Also cross-referenced as UMIACS-TR-95-81

A Flexible Traffic Shaper for High Speed Networks: Design and Comparative Study with Leaky Bucket

Maximizing bandwidth utilization and providing performance guarantees, in the context of multimedia networking, are two incompatible goals. Heterogeneity of the multimedia sources calls for effective traffic control schemes to satisfy their diverse Quality of Service (QoS) requiremnets. These include admission control at connection set up, traffic control at the source ends and efficient scheduling schemes at the switches. The emphasis in this paper is on traffic control at the source end. Most multimedia sources are bursty in nature. Traffic shapers have been mainly studied hitherto from the point of view of their effectiveness in smoothing the burstiness. Leaky Bucket (LB) scheme, to cite an example, is a mean rate policer smoothing at the token generation rate. Studies on bursty sources show that burstiness promotes statistical multiplexing at the cost of possible congestion. Smoothing, on the other hand, helps in providing guarantees at the cost of utilization. Thus need for a flexible scheme which can provide a reasonable compromise between utilization and performance is imminent. Recent studies [10, 12] have also questioned the suitability of LB for policing real-time traffic due to the excessive delays. We argue for a policy which is less stringenton short term burstiness than the LB. We propose a new traffic shaper which can adjust the burstiness of the input traffic to obtain reasonable bandwidth utilization while maintaining statistical service guarantees. The performance study is conducted in two parts. In the first part, we study the effect of varying the shaper parameters on the input characteristics. In the second part, we dimension our scheme and a LB equivalently and compare the mean and peak rate policing behavior with delay and loss as the performance parameters. Adopting a less stringent attitude towards short term burstiness is shown to result in considerable advantage while policing real-time traffic. Future research possibilities in this topic are explored. (Also cross-referenced as UMIACS-TR-95-71

A Generic Architecture for Programmable Traffic Shaper for High Speed Networks

Traffic shapers by preventing congestion and smoothing the traffic, play an important role in realizing the traffic control schemes employed in high speed networks to ensure the Quality of Service (QoS) requirements of the application. In this report, we present a generic architecture for programmable traffic shaper for high speed networks. The programmability of the proposed architecture is illustrated by implementing some of the existing traffic shaping schemes. The architectural design issues of the proposed scheme are described and discussed. (Also cross-referenced as UMIACS-TR-95-75

Design & Performance Study of a Flexible Traffic Shaper for High Speed Networks

In networks supporting distributed multimedia, maximizing bandwidth utilization and providing performance guarantees are two incompatible goals. Heterogeneity of the multimedia sources calls for effective congestion control schemes to satisfy the diverse Quality of Service (QoS) requirements of each application. These include admission control at connection set up, traffic control at the source ends and efficient scheduling schemes at the switches. The emphasis in this paper is on traffic control at the source ends. Traffic control schemes have two functional roles. One is traffic enforcement as a supplement to the admission control policy. The other is shaping the input traffic so that it becomes amenable to the scheduling mechanism at the switches for providing the required QoS guarantees. Studies on bursty sources have shown that burstiness promotes statistical multiplexing at the cost of possible congestion. Smoothing the traffic helps in providing guarantees at the cost o f bandwidth utilization. The need for a flexible scheme which can provide a reasonable compromise between the utilization and guarantees is imminent. We present the design and performance study of a flexible traffic shaper which can adjust the burstiness of input traffic to obtain reasonable utilization while maintaining statistical service guarantees. The performance of the traffic shaper for bursty sources is studied using simulation. (Also cross-referenced as UMIACS-TR-95-72

NetDyn Revisited: A Replicated Study of Network Dynamics

In 1992 and 1993, a series of experiments using the NetDyn tool was run at the University of Maryland to characterize network behavior. These studies identified multiple design and implementation faults in the Internet. Since that time, there has been a wide array of changes to the Internet. During the Spring of 1996, we conducted a replication of the NetDyn experiments in order to characterize end-to-end behavior in the current environment. In this paper, we present and discuss the latest results obtained during this study. Although the network seems to be stabilizing with respect to transit times, our current results are similar to the results from past experiments. That is, networks often exhibit unexpected behavior. The data suggest that while there has been improvement, there are still problem areas that need to be addressed. (Also cross-referenced as UMIACS-TR-96-69

AD (Attacker Defender) Game

Information Dynamics is a framework for agent-based systems that gives a central position to the role of information, time, and the value of information. We illustrate system design in the Information Dynamics Framework by developing an intelligence game called AD involving attackers, defenders and targets operating in some space of locations. The goal of the attackers is to destroy all targets. Target destruction takes place when the number of attackers in the target's neighborhood exceeds the number of defenders in this neighborhood by a value WINNING_DIFFERENCE. The goal of defenders is to prevent attackers from achieving their goal. (Also UMIACS-TR-2001-45

The Challenges of Real-Time AI

The research agendas of two major areas of computer science are converging: Artificial Intelligence (AI) methods are moving towards more realistic domains requiring real-time responses, and real-time systems are moving towards more complex applications requiring intelligent behavior. Together, they meet at the crossroads of interest in "real-time intelligent control," or "real-time AI." This subfield is still being defined by the common interests of researchers from both real-time and AI systems. As a result, the precise goals for various real-time AI systems are still in flux. This paper describes an organizing conceptual structure for current real-time AI research, clarifying the different meanings this term has acquired for various researchers. Having identified the various goals of real-time AI research, we then specify some of the necessary steps towards reaching those goals. This in turn enables us to identify promising areas for future research in both AI and real-time systems techniques. (Also cross-referenced as UMIACS-TR-94-69