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A Little Bit of Real-Time Java

A Little Bit of Real-Time Java. Nels Beckman SSSG November 7 th , 2005. Introduction. Design choices, trade-offs and motivations in the Real-Time Specification for Java Effects of these choices Possible high-level research directions. Outline. Introduction RTSJ Background

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A Little Bit of Real-Time Java

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  1. A Little Bit of Real-Time Java Nels Beckman SSSG November 7th, 2005

  2. Introduction • Design choices, trade-offs and motivations in the Real-Time Specification for Java • Effects of these choices • Possible high-level research directions Nels Beckman, SSSG

  3. Outline • Introduction • RTSJ Background • Seven Enhanced Areas • New Memory Model • Comments on RTSJ • Conclusion Nels Beckman, SSSG

  4. RTSJ Background Briefly: What is Real-Time? • “A real-time system is one that has performance deadlines on its computations and actions n” • Real-time software is • Event-driven, Time-Driven, or both • Often embedded • Often safety-critical • Example: • Flow-Rate Control System3 Nels Beckman, SSSG

  5. RTSJ Background What is the RTSJ? • Standard Java is not appropriate for RT • It provides additions to Java, so it can be used in the development of real-time systems • Specifically • Extensions to JVM Specification • Creation of real-time API Nels Beckman, SSSG

  6. RTSJ Background Why Real-Time Java? • Java = Slow? • Real-Time = Fast? • Real-time applications typically are a mixture of real-time and non real-time threads1 • So what about the “rest” of the code in a RT application? Nels Beckman, SSSG

  7. RTSJ Background History of the RTSJ • Core requirements originate from NIST workshop • The first Java Specification Request in the Java Community Process (1999) • Proposed by IBM • The Real-Time Java Experts Group was chartered with leading its creation • Members of Note • Greg Bollella • Ben Brosgol • James Gosling Nels Beckman, SSSG

  8. Applicability to Particular Java Environments Backward Compatibility Write once, Run Anywhere Current Practice/Advanced Features Predictable Execution No syntactic extension Allow Variation in Implementation Decisions RTSJ Background RTJEG’s Guiding Principles2 Nels Beckman, SSSG

  9. Outline • Introduction • RTSJ Background • Seven Enhanced Areas • New Memory Model • Comments on RTSJ • Conclusion Nels Beckman, SSSG

  10. 7 Enhanced Areas 1. Thread Scheduling & Dispatching • Original Java Spec: “…threads with higher priority are generally executed in preference to threads with lower priority…” • RTSJ • Requires at least a fixed priority, preemptive scheduler. • Defines classes for execution eligibility and schedule feasibility. Nels Beckman, SSSG

  11. 7 Enhanced Areas 2. Synchronization • RTSJ adds: • Priority queuing on synchronized blocks • Required priority inversion avoidance • Wait free queues Nels Beckman, SSSG

  12. 7 Enhanced Areas 3. Asynchronous Event Handling • Real-time applications typically must respond to events in the real world and in other parts of the application • RTSJ Adds: • Asynchronous events • Asynchronous event handlers • Handlers are runnable, and thus are associated with deadline information, etc. • Events include periodic and one shot timers Nels Beckman, SSSG

  13. 7 Enhanced Areas 4. Asynch. Transfer of Control • Occasionally necessary to transfer control immediately and permanently • RTSJ adds ATC • Code declares itself interruptible (think ‘throws’). • Asynchronous events can enact a transfer of control to a predetermined point. Nels Beckman, SSSG

  14. 7 Enhanced Areas 5. Asynch. Thread Termination • A more specific case of asynchronous transfer of control • Java • thread.stop(),thread.destroy() • Deprecated, can lead to deadlock • RTSJ • thread.interrupt() • Thread can catch interruption and clean up Nels Beckman, SSSG

  15. 7 Enhanced Areas 6. Physical Memory Access • RTSJ Adds: • Direct access to physical memory • You specify the address range • Individual bytes, ints, longs, etc. can be accessed Nels Beckman, SSSG

  16. 7 Enhanced Areas 7. Memory Management • Java has garbage collection • Garbage collection is certainly easy to use • But is it real-time appropriate? • State-of-the-art in real-time garbage collection4 • ~50% cpu overhead • 1.5X memory overhead. Nels Beckman, SSSG

  17. Memory Management So No Heap? Now What? • RTSJ has multiple thread types • Thread • RealtimeThread • NoHeapRealtimeThread Nels Beckman, SSSG

  18. Memory Management Three Types of Memory • Heap memory • Standard Java Heap • Immortal memory • Immortal memory, exists for the life of the application • Scoped Memory • A way to reclaim memory without a garbage collector Nels Beckman, SSSG

  19. Memory Management Scoped Memory • Scopes have fixed lifetimes • Lifetime starts here: • scopedMemArea.enter() { … } • Lifetime ends: Nels Beckman, SSSG

  20. Memory Management Scoped Memory • All calls to new inside a scope, create an object inside of that scope • When the scope’s lifetime ends, all objects within are destroyed Nels Beckman, SSSG

  21. Memory Management Scope Nesting • Scopes can be nested • scopedMemArea.enter() { ... anotherScope.enter() { } ... } Nels Beckman, SSSG

  22. Memory Management Memory Safety • Must ensure • No dangling pointers • Objects can’t refer to shorter lived objects • An RTSJ must ensure this with run-time checks heap immortal parent child Nels Beckman, SSSG

  23. Memory Management Common Scoped Memory Patterns • Scoped Loops • Used when temporary information created at each iteration of a loop • eg.) void mainLoop() { while(true) { ... }} Nels Beckman, SSSG

  24. Memory Management Common Scoped Memory Patterns • Scoped Loops • Used when temporary information created at each iteration of a loop • Now: Class Runner implements Runnable { void run() { ... }} void runLoop() { memory = new LTMemory( size,max ); myRunner = new Runner(); while(true) memory.enter( myRunner );} Nels Beckman, SSSG

  25. Memory Management Common Scoped Memory Patterns • Scoped Methods • Similar to scoped loops • Body of method runs in memory scope • Caveat: • State must be saved in a parent scope • Must take explicit care to do this Nels Beckman, SSSG

  26. Memory Management A Programmer’s Nightmare? • Programming mistakes lead to run-time errors • StateObject state; void run() { state = new StateObject(); Runner r =new Runner(state); nestedScope.enter(r); //Here is ‘state’ in my scope? //Is it in sub scope? } Nels Beckman, SSSG

  27. Memory Management What’s the Problem Here? • The particular scope in which a given object resides in is implicit • The nesting relationship of scopes is implicit • Nesting relationship can change at run-time • The RTSJ has some capability here, but still requires run-time comparisons Nels Beckman, SSSG

  28. Memory Management Some Proposed Techniques • Scoped Types for Real-Time Java, Vitek et. al. • Ownership-types for Safe Region-Based Memory Management in Real-Time Java, Boypati, et al. • Cyclone: A Safe Dialect of C • All proposed techniques integrate memory location into the type system. Nels Beckman, SSSG

  29. Memory Management Sharing Data Between Threads • More Funny Behavior • Portals provide placeholder for shared data • But scopes are deleted when no threads occupy it! • This can make sharing data hard! Nels Beckman, SSSG

  30. Memory Management Sharing Data Between Threads finalize() Nels Beckman, SSSG

  31. Memory Management Sharing Data Between Threads • So we get hacks! • The Wedge-Thread pattern • Sleeps a thread inside the memory scope in order to keep it alive • Then receiver thread wakes sleeping thread up again • This is Backhanded Memory Management! Nels Beckman, SSSG

  32. RTSJ Commentary Comments on the RTSJ • First: Designing frameworks and extensions is hard • External constraints: • eg.) Syntax should stay the same to preserve tool compatibility • Finding balance between users’ previous experience, and what the new domain mandates • eg.) We would like to use garbage collection, but performance is unacceptable Nels Beckman, SSSG

  33. RTSJ Commentary Comments on the RTSJ • Tendencies are hard to break? • Java tends to push errors towards run-time discovery • But this philosophy may be counter to Real-Time • eg.) When my real-time app is running, what do I do with a MemoryAccessError? What about a ScopeCycleException? • Designers of real-time applications don’t like to be surprised! Nels Beckman, SSSG

  34. RTSJ Commentary Comments on the RTSJ • Would real-time developers use the RTSJ? • I say it depends on the task • In safety critical systems, for the most part, memory is not dynamically allocated. • But realistically, RTSJ seems targeted at soft real-time, or at least non critical. Nels Beckman, SSSG

  35. RTSJ Commentary Proposed Research Directions • Soft real-time is an interesting problem space • Can currently ‘interactive’ software, be given some real-time guarantees? • Appliances, media decoders, etc. • Can we make it cheaper to do? • Hopefully, type systems and annotations may provide the answer! Nels Beckman, SSSG

  36. RTSJ Commentary Proposed Research Directions • Currently, reading up on type systems that help us to bound resource usage Nels Beckman, SSSG

  37. Conclusion: • Thanks for your attention • I hoped you learned something about Real-Time/RTSJ! Nels Beckman, SSSG

  38. References • Bollella, G., Canham, T., Carson, V., Champlin, V., Dvorak, D., Giovannoni, B., Indictor, M., Meyer, K., Murray, A., and Reinholtz, K. 2003. Programming with non-heap memory in the real time specification for Java. In Companion of the 18th Annual ACM SIGPLAN Conference on Object-Oriented Programming, Systems, Languages, and Applications (Anaheim, CA, USA, October 26 - 30, 2003). OOPSLA '03. ACM Press, New York, NY. • Bollella, G.; Gosling, J. The real-time specification for JavaComputer, Vol.33, Iss.6, Jun 2000 Pages:47-54 • Douglass, B. P. 1999 Doing hard time: developing real-time systems with UML, objects, frameworks, and patterns. Addison-Wesley Longman Publishing Co., Inc. • Tian Zhao, James Noble, Jan Vitek. "Scoped Types for Real-Time Java," rtss, pp. 241-251,  25th IEEE International Real-Time Systems Symposium (RTSS'04),  2004. • Pizlo F, Fox JM, Holmes D, Vitek J. “Real-Time Java Scoped Memory: Design Patterns and Semantics.” Nels Beckman, SSSG

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