Static WCET Analysis vs. Measurement: What is the Right Way to Assess Real-Time Task Timing? Worst Case Execution Time Prediction by Static Program Analysis Is your program always fast enough? Christian Ferdinand AbsInt Angewandte Informatik GmbH

2  Controllers in planes, cars, plants, … are expected to finish their tasks within reliable time bounds.  Schedulability analysis must be performed  Hence, it is essential that an upper bound on the execution times of all tasks is known  Commonly called the Worst-Case Execution Time (WCET) Hard Real-Time Systems

3 The Timing Problem P r o b a b i l i t y Execution Time Best Case Execution Time Exact Worst Case Execution Time SafeWorst Case Execution Time Estimate Unsafe: Execution Time Measurement

4 Embedded Control Software  Tends to be large and complex  Much functionality  Code generator tools  3 rd party software  RTOS  communication libraries

5 The Timing Problem LOAD r2, _a LOAD r1, _b ADD r3,r2,r1 MPC 5xx (2000) PPC 755 (2001) x = a + b; 68K (1990)

6 Some Architectural Features that make Measurement-Based WCET Analysis a Challenge  The empty cache is not necessarily the “worst case cache”  “Domino” effects  A cache miss is not necessarily the worst case

7 Solution: Static WCET Analysis  The WCET analyzer computes safe upper bounds of the execution times of the tasks in a program for all inputs  Static program analysis based on Abstract Interpretation  The analysis design is proven to be correct

8  Combines global program analysis by abstract interpretation for cache, pipeline, and value analysis with integer linear programming for path analysis in a single intuitive GUI. aiT WCET Analyzer A Solution to the Timing Problem

9 aiT WCET Analyzer Advantages  aiT WCET analyzer allows you to:  inspect the timing behavior of (timing critical parts of) your code  The analysis results  are determined without the need to change the code  hold for all inputs and all executions (for the intrinsic cache and pipeline behavior)

10 Conclusion  Static WCET analysis enables development of complex hard real-time systems on state-of- the-art hardware.  Increases safety.  Saves development time.  Usability proven in industrial practice.

11 When to Use Runtime Measurements?  Offline:  To validate the pipeline model wrt. the real hardware  Online (e.g., in the scheduler):  To monitor the correct working (detect hardware failures, …)  To improve the exploitation of resources  …