WCET Analysis for a Java Processor

1. WCET Analysis for a Java Processor Martin Schoeberl TU Vienna, Austria Rasmus Pedersen CBS, Denmark

2. Outline • Motivation • WCET analysis • The Java processor JOP • Results • Conclusion, future work • Demo Martin Schoeberl – WCET Analysis for a Java Processor

3. Motivation • Schedule analysis • For (hard) real-time • Execution time numbers needed • Static WCET analysis • No measurements! • Can not guarantee the WC • Still used Martin Schoeberl – WCET Analysis for a Java Processor

4. Issues with static WCET Analysis • Why is WCET Analysis so seldom used? • High-level part is easy • Low-level is the hard part • Instruction timing • Caches • Advanced, speculative processors Martin Schoeberl – WCET Analysis for a Java Processor

5. Static WCET Analysis • Mature research • High-level based in ILP • Construct CFG • Add execution time to BB • Build ILP equations: • Sum of BB texe will be maximized • Frequency of in-edges = out-edges • Add loop constraints Martin Schoeberl – WCET Analysis for a Java Processor

6. Low-level WCET Analysis • Execution time of basic blocks • Sum of instruction timing? • Not in modern CPUs • What about memory access? • Instruction cache • Instruction prefetch • Data cache • Very hard for general purpose CPUs Martin Schoeberl – WCET Analysis for a Java Processor

7. The Proposed Solution • Build a processor to simplify WCET Analysis • Avoid non analyzable features • Find better solutions • Java processor JOP • Built from ground up for WCET • FPGA implementation (small) • Not slow on average Martin Schoeberl – WCET Analysis for a Java Processor

8. JOP Features • A RISC stack machine • 4 stage pipeline • No dependencies • No shared state (e.g. memory) • Stack cache • Method cache Martin Schoeberl – WCET Analysis for a Java Processor

9. Size Martin Schoeberl – WCET Analysis for a Java Processor

10. Performance Martin Schoeberl – WCET Analysis for a Java Processor

11. Low-level Timing • Bytecode execution time known • Analysis of microcode (DATE’06) • No dependencies • Documented Martin Schoeberl – WCET Analysis for a Java Processor

12. Memory Access • Assume SRAM • Constant access time (rws and wws) • Access time partially hidden: • Method cache load: • Partially hidden Martin Schoeberl – WCET Analysis for a Java Processor

13. Method Cache • Full method loaded • Misses only on invoke/return • Cache contains several methods • Simpler to analyze • At call tree level • Other instructions are a hit Martin Schoeberl – WCET Analysis for a Java Processor

14. Method Cache Analysis • Only call tree leaves • Return is always a hit • Invoke in a loop for (int i=0; i<10; ++i) { foo(); } • One miss and 9 hits • Miss times added to the CFG Martin Schoeberl – WCET Analysis for a Java Processor

15. Miss Times in the CFG => Martin Schoeberl – WCET Analysis for a Java Processor

16. Evaluation Martin Schoeberl – WCET Analysis for a Java Processor

17. Conclusion • We need static WCET analysis • COTS processors don’t work • Design HW for WCET • A RT computer architecture • JOP is a first step to RT processors • Analysis at bytecode level Martin Schoeberl – WCET Analysis for a Java Processor

18. Future Work • Method cache cont. • Detection of loop bounds • Integration into Eclipse • WC memory consumption Martin Schoeberl – WCET Analysis for a Java Processor

19. Demo Time Martin Schoeberl – WCET Analysis for a Java Processor

20. Thank You! Questions & Suggestions Martin Schoeberl – WCET Analysis for a Java Processor