Concurrent and RealTime Programming in Java Electronic copies

Concurrent and Real-Time Programming in Java § Electronic copies of course foils available via Ø http: //www-course. cs. york. ac. uk/crt § Course book: “Concurrent and Real-Time Programming in § Java” Andy Wellings, Wiley, 2004 (£ 20. 99 from Amazon) Real-Time Specification for Java (RTSJ) Versions 1. 0. 1 is available from Ø http: //www. rtj. org © Andy Wellings, 2004

Books RTSJ Version 0. 9 © Andy Wellings, 2004 RTSJ Version 1. 0. 1

Other books RTSJ Version 1. 0 © Andy Wellings, 2004 RTSJ Version 0. 9

Practicals § Start week 3 § Demonstrators: © Andy Wellings, 2004

Thursday 14. 15 © Andy Wellings, 2004

Thursday 15. 15 © Andy Wellings, 2004

Prerequisites § You should already: Ø be a competent programmer in an imperative programming language like C, Pascal, Ada, C++, C# etc Ø be able to program in sequential Java Ø have a good understanding of Operating System Principles, in particular the mechanisms needed to support concurrency, e. g. processes, semaphores, etc © Andy Wellings, 2004

Overall Technical Aims of the Course § To understand the basic requirements of concurrent and real-time systems § To understand how these requirements have influenced the design of Java and the Real-Time Specification for Java § To be able to program advanced concurrent real-time Java systems © Andy Wellings, 2004

Course Contents I § § § § Introduction to Course, Concurrent and Real-Time Programming Concurrent Programming in Java Communication and Synchronization Motivation for, and an overview of, the RTSJ Memory Management Clocks and Time Scheduling and Schedulable Objects Asynchronous Events and Handlers © Andy Wellings, 2004

Course Contents II § § § Real-Time Threads Asynchronous Transfer of Control Resource Control Schedulability Analysis Conclusions © Andy Wellings, 2004

Concurrent Programming § The name given to programming notation and techniques for expressing potential parallelism and solving the resulting synchronization and communication problems § Implementation of parallelism is a topic in computer systems (hardware and software) that is essentially independent of concurrent programming § Concurrent programming is important because it provides an abstract setting in which to study parallelism without getting bogged down in the implementation details © Andy Wellings, 2004

Why we need it Response time in seconds § To fully utilise the processor 10 2 1 10 0 10 -1 10 human floppy -2 10 -3 10 -4 CD 10 -5 10 -6 10 -7 10 -8 10 10 tape memory processor -9 © Andy Wellings, 2004

Parallelism Between CPU and I/O Devices CPU I/O Device Initiate I/O Operation Process I/O Request Signal Completion Interrupt I/O Routine I/O Finished © Andy Wellings, 2004 Continue with Outstanding Requests

Why we need it § To allow the expression of potential parallelism so that more than one computer can be used to solve the problem § Consider trying to find the way through a maze © Andy Wellings, 2004

Sequential Maze Search

Concurrent Maze Search

Why we need it § To model the parallelism in the real world § Virtually all real-time systems are inherently concurrent — § devices operate in parallel in the real world This is, perhaps, the main reason to use concurrency © Andy Wellings, 2004

Air Traffic Control

Why we need it § Alternative: use sequential programming techniques § The programmer must construct the system as the cyclic § § § execution of a program sequence to handle the various concurrent activities This complicates the programmer's task and involves considerations of structures which are irrelevant to the control of the activities in hand The resulting programs will be more obscure and inelegant Decomposition of the problem is more complex Parallel execution of the program on more than one processor is more difficult to achieve The placement of code to deal with faults is more problematic © Andy Wellings, 2004

Terminology § A concurrent program is a collection of autonomous sequential processes, executing (logically) in parallel § Each process has a single thread of control § The actual implementation (i. e. execution) of a collection of processes usually takes one of three forms. Multiprogramming Ø processes multiplex their executions on a single processor Multiprocessing Ø processes multiplex their executions on a multiprocessor system where there is access to shared memory Distributed Processing Ø processes multiplex their executions on several processors which do not share memory © Andy Wellings, 2004

What is a real-time system? § A real-time system is any information processing system which has to respond to externally generated input stimuli within a finite and specified period Ø the correctness depends not only on the logical result but also the time it was delivered Ø failure to respond is as bad as the wrong response! § The computer is a component in a larger engineering system => EMBEDDED COMPUTER SYSTEM § 99% of all processors are for the embedded systems market © Andy Wellings, 2004

Terminology § Hard real-time — systems where it is absolutely imperative that responses occur within the required deadline. E. g. Flight control systems. § Soft real-time — systems where deadlines are important but which will still function correctly if deadlines are occasionally missed. E. g. Data acquisition system. § Firm real-time — systems which are soft real-time but in which there is no benefit from late delivery of service. A system may have all hard, soft and real-time subsystems Many systems may have a cost function associated with missing each deadline © Andy Wellings, 2004

A simple fluid control system Interface Pipe Input flow reading Flow meter Processing Output valve angle Time Computer © Andy Wellings, 2004 Valve

A Grain-Roasting Plant Bin Furnace Fuel Tank grain Pipe fuel © Andy Wellings, 2004

A Process Control System Process Control Computer Chemicals and Materials Valve Temperature Transducer PLANT © Andy Wellings, 2004 Stirrer Finished Products

A Production Control System Finished Products Parts Machine Tools © Andy Wellings, 2004 Manipulators Conveyor Belt

A Command Control System Command Post Command Control Computer Temperature, Pressure, Power and so on Terminals © Andy Wellings, 2004 Sensors/Actuators

A Typical Embedded System Real-Time Clock Algorithms for Digital Control Interface Engineering System Data Logging Remote Monitoring System Data Retrieval and Display Devices Database Operator’s Console © Andy Wellings, 2004 Operator Interface Real-Time Computer

Characteristics of a RTS § Large and complex — vary from a few hundred lines of § § assembler or C to 20 million lines of Ada estimated for the Space Station Freedom Concurrent control of separate system components — devices operate in parallel in the real-world; better to model this parallelism by concurrent entities in the program Facilities to interact with special purpose hardware — need to be able to program devices in a reliable and abstract way © Andy Wellings, 2004

Characteristics of a RTS § Extreme reliability and safe — embedded systems typically § control the environment in which they operate; failure to control can result in loss of life, damage to environment or economic loss Guaranteed response times — we need to be able to predict with confidence the worst case response times for systems; efficiency is important but predictability is essential © Andy Wellings, 2004

Real-time Programming Languages § Assembly languages § Sequential systems implementation languages — e. g. § § § RTL/2, Coral 66, Jovial, C. Both normally require operating system support. High-level concurrent languages. Impetus from the software crisis. e. g. Ada, Chill, Modula-2, Mesa, Java. No operating system support! We will focus on Java and the Real-Time Specification for Java See Burns, Wellings, Real-Time Systems and Programming Languages, 3 rd Edition, 2001, Addison Wesley for a general discussion on other languages and operating systems © Andy Wellings, 2004

Java Architecture JBC JBC Hardware JVM Standalone JVM Real-time Operating System © Andy Wellings, 2004 JVM

Summary I § The motivations for concurrent programming have been presented: fully utilize the processor Ø allow parallel execution Ø model real world parallelism Ø § Two main classes of real-time systems have been identified: hard real-time systems Ø soft real-time systems Ø © Andy Wellings, 2004

Summary II § The basic characteristics of a real-time or embedded computer system are: Ø Ø Ø Ø largeness and complexity, manipulation of real numbers, extreme reliability and safety, concurrent control of separate system components, real-time control, interaction with hardware interfaces, efficient implementation. © Andy Wellings, 2004

Further Reading § Chapter 1 of Burns and Wellings, “Real-Time Systems and Programming Languages”, 3 rd Edition, 2001 (in library) © Andy Wellings, 2004