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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. Hard Real-Time Systems. - PowerPoint PPT Presentation
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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 ProblemPr
obab
ility
Execution Time
Best CaseExecution Time
Exact Worst CaseExecution Time
Safe Worst Case Execution TimeEstimate
Unsafe: Execution TimeMeasurement
4
Embedded Control Software
Tends to be large and complex Much functionality Code generator tools 3rd party software
RTOS communication libraries
5
The Timing Problem
LOAD r2, _a
LOAD r1, _b
ADD r3,r2,r1
0
10
20
30
0 WaitCycles
1 WaitCycle
External(6,1,1,1,...)
Execution Time depending on Flash Memory(Clock Cycles)
Clock Cycles
0
50
100
150
200
250
300
350
Best Case Worst Case
Execution Time (Clock Cycles)
Clock Cycles
MPC 5xx (2000) PPC 755 (2001)
x = a + b;
0
50
100
150
200
250
300
350
Best Case Worst Case
Execution Time (Clock Cycles)
Clock Cycles
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 AnalyzerA Solution to the Timing Problem
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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.
11When 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 …
email: [email protected]://www.AbsInt.com
