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Toward Deterministic Java Performance
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  1. Toward Deterministic Java Performance Mark Stoodley, Mike Fulton Toronto Lab, IBM Canada Ltd. Compilation Technology

  2. The Real-Time World • Responsivein “real time” • Often keyed to real world events • Performing work on a regular basis • Asynchronous events • Graceful handling of truly exceptional conditions • Deterministic performance key to meet response time requirements • Java performance not really responsive as-is • But it’s a nice development environment • Motivates the Real-Time Specification for Java Compilation Technology

  3. Overview • Real-Time Java • Java performance isn’t really deterministic  • Mitigating the Chaos • Summary Compilation Technology

  4. Real-Time Java • JSR #1: Real-Time Specification for Java • Facilities to support Real-Time programming • Make performance more controllable & predictable • Large-scale enhancements to Java • Threading, scheduling • Memory management • Asynchronous event handling, control transfer, termination • Physical memory access Compilation Technology

  5. Example 1: Memory Management • SPEC assumes that managed memory (garbage collection) is incompatible with real-time needs • New memory areas that are not collected • Immortal memory • Memory scopes • New thread type “No Heap Realtime Thread” • Not permitted to even see a heap reference • No need to stop for any reason when GC occurs Compilation Technology

  6. Java performance isn’t really deterministic • Chaos lurks everywhere: • Thread scheduling is at the whim of the operating system • Garbage collection occurs “whenever” for “however long” • JIT compilations occur “whenever” for “however long” • Aggressive JITs recompile methods that seem “hot” • JIT compilers employ many speculative optimizations • Class loading occurs on demand Compilation Technology

  7. Mitigating the JIT Chaos: stop doing “bad” stuff • JIT compiling delays are unacceptable • Also derivative effects: profiling, sampling • Could run at low priority BUT risk priority inversion • Ahead-of-Time (AOT) compilation a better option • Takes compiler out of the run-time performance equation • Possibly lower performance to deal with resolution order • Derivative effects also removed • BUT maybe more difficult to achieve high performance Compilation Technology

  8. Mitigating the JIT Chaos: stop doing “bad” stuff • Stop doing speculative optimizations • No flat-word monitors • Also simplifies priority-inversion support • No monitor coarsening • Profiling-based optimizations • Not easy because JIT compilers speculate a LOT • Devirtualization ok if all classes are pre-loaded Compilation Technology

  9. Mitigating the JIT Chaos: stop doing “bad” stuff • Class loading is a trouble spot • Loading one class often requires loading other classes • Once class is loaded, devirtualizations may be invalid • Lots of call sites may need to be patched for correctness • Updates many VM data structures also accessed by GC • Particularly a problem for NoHeapRealtimeThreads • Application-level pre-loading is one option • Collect list of loaded classes in one execution • “Force” class to load before application begins executing Compilation Technology

  10. Summary • Java not suitable as-is for Real-Time workloads • Real-Time Specification enhances Java for RT • Java VMs have many sources of nondeterminism • GC, thread scheduling, JIT compiler • These problems can be largely mitigated • Ahead-of-Time compiles, class preloading, stop doing speculative optimizations • Lower sustained performance but more deterministic Compilation Technology

  11. Contact Information and Acknowledgments Mark Stoodley Toronto Lab, IBM Canada Ltd. mstoodle@ca.ibm.com With thanks to: Mike Fulton Compilation Technology

  12. Backup Slides Compilation Technology

  13. Example 2: Asynchronous Transfer of Control (ATC) • RT programs need to respond to truly exceptional conditions quickly and drastically • Thread that detects condition may need to interrupt other threads actively working • ATC provides facilities to mark methods that can be safely interrupted • More draconian exception semantics in such methods • Also mechanisms to initiate such interruptions Compilation Technology