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Concurrent and Real-Time Programming in Java

This course provides an introduction to concurrent and real-time programming in Java, covering topics such as communication, synchronization, memory management, scheduling, and asynchronous events. It also explores the Real-Time Specification for Java (RTSJ).

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Concurrent and Real-Time Programming in Java

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  1. Concurrent and Real-Time Programming in Java • Electronic copies of course foils available via • 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 • http://www.rtj.org

  2. RTSJ Version 0.9 Books RTSJ Version 1.0.1

  3. Other books RTSJ Version 1.0 RTSJ Version 0.9

  4. Practicals • Start week 3 • Demonstrators:

  5. Thursday 14.15

  6. Thursday 15.15

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

  8. 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

  9. 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

  10. Course Contents II • Real-Time Threads • Asynchronous Transfer of Control • Resource Control • Schedulability Analysis • Conclusions

  11. 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

  12. memory processor human floppy tape CD -1 -2 -9 -3 -4 -8 -5 -6 -7 1 2 0 10 10 10 10 10 10 10 10 10 10 10 10 Why we need it • To fully utilise the processor Response time in seconds

  13. CPU I/O Device Initiate I/O Operation Process I/O Request Signal Completion Interrupt I/O Routine I/O Finished Continue with Outstanding Requests Parallelism Between CPU and I/O Devices

  14. 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

  15. Sequential Maze Search

  16. Concurrent Maze Search

  17. 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

  18. Air Traffic Control

  19. 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

  20. 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 • processes multiplex their executions on a single processor Multiprocessing • processes multiplex their executions on a multiprocessor system where there is access to shared memory Distributed Processing • processes multiplex their executions on several processors which do not share memory

  21. 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 • the correctness depends not only on the logical result but also the time it was delivered • 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

  22. 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 real-time subsystems Many systems may have a cost function associated with missing each deadline

  23. A simple fluid control system Interface Pipe Input flow reading Flow meter Processing Valve Output valve angle Time Computer

  24. A Grain-Roasting Plant Bin Furnace Fuel Tank grain Pipe fuel

  25. A Process Control System Process Control Computer Temperature Transducer Finished Products Valve Stirrer Chemicals and Materials PLANT

  26. A Production Control System Production Control System Finished Products Parts Machine Tools Manipulators Conveyor Belt

  27. A Command and Control System Command Post Command and Control Computer Temperature, Pressure, Power and so on Terminals Sensors/Actuators

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

  29. 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

  30. 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

  31. 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, 3rd Edition, 2001, Addison Wesley for a general discussion on other languages and operating systems

  32. Java Architecture JBC JBC JBC JVM Hardware JVM Standalone JVM Real-time Operating System

  33. Summary I • The motivations for concurrent programming have been presented: • fully utilize the processor • allow parallel execution • model real world parallelism • Two main classes of real-time systems have been identified: • hard real-time systems • soft real-time systems

  34. Summary II • The basic characteristics of a real-time or embedded computer system are: • 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.

  35. Further Reading • Chapter 1 of Burns and Wellings, “Real-Time Systems and Programming Languages”, 3rd Edition, 2001 (in library)

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