SYNCHRONIZATION IN REAL-TIME SYSTEMS. A Priority Inheritance Approach

Transcription

1 SYNCHRONIZATION IN REAL-TIME SYSTEMS A Priority Inheritance Approach

2 THE KLUWER INTERNATIONAL SERIES IN ENGINEERING AND COMPUTER SCIENCE REAL-TIME SYSTEMS Consulting Editor John A. Stankovic REAL TIME UNIX SYSTEMS: Design and Application Guide, B. Furht, D. Grostick, D. Gluch, G. Rabbat, J. Parker, M. McRoberts, ISBN: FOUNDATIONS OF REAL TIME COMPUTING: Scheduling and Resource Management, A. M. van Tilborg, G. M. Koob ISBN: FOUNDATIONS OF REAL TIME COMPUTING: Formal Specifications and Methods, A. M. van Tilborg, ISBN: CONSTRUCTING PREDICTABLE REAL TIME SYSTEMS, W. A. Halang, A. D. Stoyenko ISBN:

3 SYNCHRONIZATION IN REAL-TIME SYSTEMS A Priority Inheritance Approach by Ragunathan R2\ikumar IBM, Thomas J. Watson Research Center ~. " SPRINGER SCIENCE+BUSINESS MEDIA, LLC

4 Library of Congress Cataloging-in-Publieation Data Rajkumar, Ragunathan. Synchronization in real-time systems : a priority inheritance approach / by Ragunathan Rajkumar. p. cm. -- (The Kluwer international series in engineering and computer science ; 151. Real-time systems) Includes bibliographical references and index. ISBN l ISBN (ebook) DOI / Real-time data processing. 1. Title. II. Series: Kluwer international series in engineering and computer science ; SECS 151. III. Series: Kluwer international series in engineering and computer science. Real-time systems. QA76.54.R '.33--dc CIP Copyright e 1991 Springer Science+Business Media New York Originaily published by Kluwer Academic Publishers in 1991 Softcover reprint ofthe hardcover Ist edition 1991 AII rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, mechanical, photo-copying, recording, or otherwise, without the prior written permission of the publisher, Springer Science+ Business Media, LLC. Printed an acid-free paper.

5 To my Parents

6 Table of Contents List of Figures List of Tables Preface Acknowledgments xi xiii xv xvii Chapter One: Introduction Real-Time Systems Scheduling in Real-Time Systems Related Work Scope of This Book Approach Taken Organization ofthe Book 12 Chapter Two: Real-Time Synchronization in Uniproces- 15 sors 2.1 Introduction Priority Inheritance Protocols The Basic Priority Inheritance Protocol The Definition of The Basic Protocol Implementing the Basic Priority Inheritance Protocol Properties of the Basic Priority Inheritance Protocol The Priority Ceiling Protocol Overview Definition of the Priority Ceiling Protocol An Implementation of the Priority Ceiling Protocol The Properties of The Priority Ceiling Protocol 34

7 viii 2.5 The Semaphore Control Protocol Definition of the Semaphore Control Protocol Schedulability Analysis of Priority Inheritance Protocols The Impact of Priority Inversion on Schedulability Determination of Bi for SCP Determination of Bi for PCP Determination of Bi for the Basic Inheritance Protocol Extensions to the Priority Inheritance Protocols Performance of the Priority Inheritance Protocols Deadlock Avoidance Protocol Comparison Under Transient Overloads Summary of Results of Experimental Studies Summary 58 Chapter Three: Synchronization in Multiple Processor 61 Systems 3.1 Introduction Synchronization in Multiple Processor Systems Typical System Configuration Static Binding vs. Dynamic Binding The Concept of Remote Blocking Assumptions and Notation Global and Local Priority Ceilings The Distributed Priority Ceiling Protocol Synchronization Processors Normal Execution Priority ofa GCS Terminology Illustration Implementation Considerations Illustration ofthe Distributed Protocol The Definition of the Distributed Priority Ceiling Protocol Effects of Deferred Execution Properties of the Distributed Priority Ceiling Protocol Schedulability Analysis A Schedulability Analysis Example Task Allocation The Multiprocessor Priority Ceiling Protocol Differences with the Distributed Protocol The Definition of the Multiprocessor Priority Ceiling 99 Protocol Determination of Task Blocking Times Schedulability Analysis 108

8 ix A Schedulability Analysis Example Comparison of Multiple Processor Synchronization Protocols Implementation Considerations Variations in the Multiprocessor Priority Ceiling Protocol Summary 11 7 Chapter Four: Distributed Real-Time Databases Introduction Motivation Related Work Real-Time Concurrency Control Issues The Priority Inversion Problem The Read-Write Priority Ceiling protocol Extending the rw_priority Ceiling Protocol Distributed Database Issues Summary 139 Chapter Five: Conclusion Summary of Results Directions for Future Research Concluding Remarks 147 Appendix A: Computing Bi For The Basic Inheritance 149 Protocol Appendix B: Notation Used 157 References 161 Index 169

9 List of Figures Figure 1-1: An Embedded Real-Time System 3 Figure 2-1: Sequence of Events described in Example Figure 2-2: Sequence of Events described in Example Figure 2-3: Sequence of Events described in Example Figure 3-1: A Typical Multiple Processor System Configuration 63 Figure 3-2: Remote Processor Blocking: Example Figure 3-3: Remote Processor Blocking: Example Figure 3-4: Task and Semaphore Allocation of Example Figure 3-5: Task and Semaphore Allocation of Example Figure 3-6: The Sequence of Events described in Example Figure 3-7: Liu and Layland's worst case 83 Figure 3-8: The new worst case with deferred execution 84 Figure 3-9: The Task Allocation Procedure of Example Figure 3-10: Task and Semaphore Allocation of Example Figure 3-11: Sequence of events described in Example Figure 4-1: The Pseudo-Code For a Tracking Transaction 123 GlobaC View. Figure 4-2: Sequence of Events described in Example Figure A-I: Simpletons 150 FigureA-2: Final Search Tree 151 FigureA-3: Maximum Weighted Path 152 FigureA-4: Search Tree for Maximum Transformed Weighted 153 Path FigureA-5: The Search Tree Dual for Maximum Transformed 155 Weighted Path

10 List of Tables Table 2 1: Breakdown Utilization for Different Protocols with 5 51 Semaphores Table 2 2: Breakdown Utilization for Different Protocols with Semaphores Table 2 3: Breakdown Utilizations from Various Ada Run 52 Time Systems Table 2 4: Deadlock A voidance Properties of Different 56 Protocols Table 2 5: Relative Deadline Misses for Different Protocols 57 with 5 semaphores Table 2 6: Deadline Misses for Different Protocols wi semaphores Table 3 1: The Priority Ceilings of Semaphores in Example Table 3 2: The Normal Execution Priorities of Critical Sections 78 in Example 3.4 Table 3 3: The Parameters of Task Set in Example Table 3 4: The Blocking Factors for Each Task in Example Table 3 5: The Schedulability Test Parameters for Each 91 Processor of Example 3.4 Table 3 6: The Priority Ceilings of Semaphores in Example Table 3 7: Normal Execution Priorities of Critical Sections in 100 Example 3.7 Table 3 8: The Execution Priorities of Global Critical Sections 110 for Section Table 3 9: The Blocking Factors for Each Task in Section Table 3 10: The Schedulability Test Parameters for Each 113 Processor of Example 3.4

11 Preface Real-time computing systems are vital to a wide range of applications. For example, they are used in the control of nuclear reactors and automated manufacturing facilities, in controlling and tracking air traffic, and in communication systems. In recent years, real-time systems have also grown larger and become more critical. For instance, advanced aircraft such as the space shuttle must depend heavily on computer systems [Carlow 84]. The centralized control of manufacturing facilities and assembly plants operated by robots are other examples at the heart of which lie embedded real-time systems. Military defense systems deployed in the air, on the ocean surface, land and underwater, have also been increasingly relying upon real-time systems for monitoring and operational safety purposes, and for retaliatory and containment measures. In telecommunications and in multi-media applications, realtime characteristics are essential to maintain the integrity of transmitted data, audio and video signals. Many of these systems control, monitor or perform critical operations, and must respond quickly to emergency events in a wide range of embedded applications. They are therefore required to process tasks with stringent timing requirements and must perform these tasks in a way that these timing requirements are guaranteed to be met. Real-time scheduling algorithms attempt to ensure that system timing behavior meets its specifications, but typically assume that tasks do not share logical or physical resources. Since resource-sharing cannot be eliminated, synchronization primitives must be used to ensure that resource consistency constraints are not violated. Unfortunately, existing mechanisms for achieving task synchronization can lead to uncontrolled priority inversion, a situation in which a higher priority job is blocked by lower priority jobs for an indefinite period of time. Unless the priority inversion problem is addressed adequately, scheduling algorithms can be of little use in realtime systems.

12 xvi Goals of This Book This book studies the principles of task synchronization in real-time systems by investigating the impact of synchronization delays on timing constraints. It develops a comprehensive set of techniques for synchronizing real-time tasks on uniprocessors, shared memory multiprocessors, distributed systems and distributed real-time databases. In particular, we investigate the class of priority inheritance protocols that solve the unbounded blocking problem, which is a generalization of the priority inversion problem. We show that there exist efficient priority inheritance protocols on uniprocessors. Since synchronization is essentially a serializing activity, the durations of blocking on shared resources become prolonged in the context of multiprocessors and distributed systems. We develop priority inheritance protocols for use on multiprocessors and distributed systems which bound the blocking duration of a task waiting for globally shared resources. We also extend the priority inheritance protocols to distinguish between the read and write semantics of locks in real-time databases. We show that compatible locks are not necessarily useful in real-time databases, but our protocol shall exploit such lock compatibility when it enhances guaranteed performance. This protocol can be extended to decomposable distributed real-time databases as well. Our primary focus in this book is upon the delay introduced by the sharing of resources. The intended goal is to bound this delay, reduce it as much as possible and account for it in the scheduling analysis. However, it is important to note that bounding the delay caused by task synchronization is not an end in itself. First and foremost, the system must be amenable to scheduling analysis in the absence of resource-sharing. The bounded delay due to resource-sharing serves only to approximate the idealized independent task scheduling model, with the net effect of making the realtime system predictable.

13 Acknowledgments Without the help and support of several people at Carnegie Mellon University, IBM Research and elsewhere, this book would not have become possible. If I now believe that I am capable of doing some research, lowe it to Dr. Lui Sha, my mentor, who introduced me to the intriguing domain of research, and taught me to question basic assumptions. Discussions, technical and otherwise, that I have had with him have always been rewarding. Thank you for everything, Lui. My eternal gratitude is also due Prof. John Lehoczky, for his incisive questioning every step of the way. His deep insight and ability to formulate research problems have never failed to amaze me. In fact, much of the work presented in this book was done with Dr. Sha and Prof. Lehoczky. My sincere thanks to Prof. Jay Strosnider, an office-mate at first, a friend then and my advisor finally! It was a pleasure to bounce ideas off him, and his amicable personality made my stay at CMU all the more enjoyable. I am grateful to Prof. Dan Siewiorek for adopting me as his student under (my) difficult circumstances, and for all the pieces of advice he has given me before and after graduation. Finally, I sincerely thank my manager at IBM Research, Dr. Robert Iannucci, and the IBM Research management who let me steal time away from my work to do this book. I also thank the Advanced Real-time Technology (ART) project members at CMU for creating the wonderful environment where work was fun. In particular, I thank Dr. Hide Tokuda, Brinkley Sprunt, Dave Kirk, Tom Marchok and especially Joan Maddamma. Dr. Andre van Tilborg was instrumental in getting me into this group. Technical discussions. at the Software Engineering Institute with the RTSIA'ns Dr. John Goodenough, Mark Borger, Mark Klein, Bob Page and Tom Ralya were always very informative. Prof. Krithi Ramamritham presented the synchronization protocol on shared memory multiprocessors at the Distributed Systems Conference on my behalf, and contributed to the development of the Semaphore Control Protocol. Mark Borger implemented the uniprocessor

14 xviii protocols on an Ada run-time system for a 68K-target and collected the test data. Mark Klein and Mark Borger pointed out that the deadline avoidance property of the priority ceiling protocol is superior to that of the ceiling semaphore protocol. The B52's (Tom Marchok, Ron Mraz, CJ Paul and Brinkley Sprunt) deserve special mention for the light-hearted office atmosphere. I also thank Bala Kumar, Gandhi Chinnadurai, Salemites, and the CMU and Pittsburgh folks who contributed more than I can acknowledge. But for my wonderful undergraduate years at PSG Tech, I would be a very different person now and none of this would have ever happened. My thanks to the entire batch of '79-'84 and in particular to all my ECE-mates. I am extremely grateful for the love, understanding, support and extreme patience shown all along by my wife Revathi. She taught me that life is much more than working at a dumb terminal. And my love goes to Vikram, for all his smiles, playfulness and curiosity. Without him being born, this book would have been written much sooner. It was certainly worth it! And thanks to my brother Mohan Kumar and sister Roop Kala for everything they had to offer during my many years at and away from home. Finally, I am grateful to my parents, especially to my late father who built in me the resolve to fight for what I wanted, and to my mother who was always there. Their relentless encouragement was perhaps the single most influential factor in my education since childhood. I dedicate this book to them.

15 SYNCHRONIZATION IN REAL-TIME SYSTEMS A Priority Inheritance Approach