Distributed real-time 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 real-time activities in distributed real-time systems often requires ad hoc methods to tune the system's runtime behavior The objective of Real-Time Mach is to develop a real-time version of the Mach kernel which provides users with a predictable and reliable distributed real-time computing environment. In this paper

Multimedia applications have timing requirements that cannot generally be satisfied using the time-sharing scheduling algorithms of general purpose operating systems. Our approach is to provide the predictability of real-time systems while retaining the flexibility of a timesharing system. We designed a processor capacity reservation mechanism that isolates programs from the timing and execution characteristics of other programs in the same way that a memory protection system isolates them from outside memory accesses. In this paper, we describe a scheduling framework that supports reservation and admission control, and we introduce a novel reserve abstraction, specifically designed for the microkernel architecture, for measuring and controlling processor usage. We have implemented processor capacity reserves in Real-Time Mach, and we describe the performance of our system on several types of applications. 1

We consider the problem of OS resource management for real-time and multimedia systems where multiple activities with different timing constraints must be scheduled concurrently. Time on a particular resource is shared among its users and must be globally managed in real-time and multimedia systems. A resource kernel is meant for use in such systems and is defined to be one which provides timely, guaranteed and protected access to system resources. The resource kernel allows applications to specify only their resource demands leaving the kernel to satisfy those demands using hidden resource management schemes. This separation of resource specification from resource management allows OS-subsystem-specific customization by extending, optimizing or even replacing resource management

This paper focuses on the problem of providing run-time support to real-time applications and non-real-time applications in an open system environment. It extends the two-level 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) real-time 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 real-time applications. 1 Introduction Recent advances in real-time systems technology have given us many good schemes for scheduling hard real-time 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...

In this paper we present five new on-line algorithms for servicing soft aperiodic requests in real-time 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 run-time overhead present nearly optimal responsiveness. A valuable contribution of this work is to provide the real-time system designer with a wide range of practical solutions which allow to balance efficiency against implementation complexity. 1 Introduction Many complex control applications include tasks which have to be completed within strict time constraint...

This paper summarizes the state of the real-time 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 table-driven scheduling, static priority preemptive scheduling, dynamic planning-based scheduling, and dynamic best efSort scheduling. In the operating system context, we argue that most of the proprietary commercial kernels as well as real-time extensions to time-sharing operating system kernels do not fit the needs of predictable real-time systems. We discuss several research kernels that are currently being built to explicitly meet the needs of real-time applications. I.

The well-known periodic task model of Liu and Layland [1] assumes a worst-case execution time bound for every task and may be too pessimistic if the worst-case execution time of a task is much longer than the average. In this paper, we give a multiframe real-time 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 N00014-94-1-0582. 1 INTRODUCTION 2 1 Introduction The well-known periodic task model by Liu and Layland(L&L) [1] assumes a worst-case execution time bound for every task. While this is a reasonable assumption for process-control-type real-time applications, it may be o...

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In a distributed system or communication network tasks may need to be executed on more than one processor. For time-critical tasks, the timing constraints are typically given as end-to-end release-times and deadlines. This paper describes algorithms to schedule a class of systems where all the tasks execute on different processors in turn in the same order. This end-to-end scheduling problem is known as the flow-shop problem. We present two cases where the problem is tractable and evaluate a heuristic for the N P-hard general case. We generalize the traditional flow-shop model in two directions. First, we present an algorithm for scheduling flow shops where tasks can be serviced more than once by some processors. Second, we describe a heuristic algorithm to schedule flow shops that consist of periodic tasks. Some considerations are made about scheduling systems with more than one flow shop. 1

This paper addresses the problem of jointly scheduling tasks with both hard and soft time constraints. We present a new analysis which builds upon previous research into slack stealing algorithms. Our analysis determines the maximum processing time which may be stolen from hard deadline periodic or sporadic tasks, without jeopardising their timing constraints. It extends to tasks with characteristics such as synchronisation, release jitter and stochastic execution times, as well as forming the basis for a family of optimal and approximate slack stealing algorithms.