MeasurementBased WorstCase Execution Time WCET Analysis

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Measurement-Based Worst-Case Execution Time
(WCET) Analysis

• Guillem Bernat and Adam Betts

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• Measurement-Based Approach the Holy Grail of WCET
  Analysis

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The real-time constraint

• Computer market dominated by embedded systems
• Majority of embedded systems operate in real-time
• Functional and Temporal correctness required
• Failure has varying degrees of consequences

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Why real-time people are useful

• In-flight control system Death
• Video streaming Degraded picture quality
• Automotive Breakdown
• Toaster Burnt toast
• Mobile phone Angry girlfriend

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Circumventing Catastrophe

• Real-Time System broken into number of tasks
• Tasks are scheduled according to their temporal
  parameters

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10
24
37
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Basics of WCET

• Safe estimates desired
• Static Analysis uses mathematical models of both
  program and processor
• These data are combined using calculation methods
• Formal proofs ascertain correctness

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instruction1 instruction2 instruction3 instruction
4 instruction5 instruction6 instruction7 instructi
on8 instruction9 instruction10
WCET 65
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instruction1 instruction2 instruction3 instruction
4 instruction5 instruction6 instruction7 instructi
on8 instruction9 instruction10 instruction11 instr
uction12 instruction13 instruction14 instruction15
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Where Static Analysis Struggles

• Processor advancements aim to improve
  average-case performance
• Predictability compromised
• Features disabled or pessimistic assumptions made
• Integration of advanced features into embedded
  domain gathering speed

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instruction1 instruction2 instruction3 instruction
4 instruction5
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It gets worse

• Interaction between all features yet to be fully
  explored
• Unrealistic assumptions about processor behaviour
• Intellectual property constrains the scope of
  static analysis

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Measure, you say?

• Scepticism toward measurement-based approach
• Pure end-to-end measurement might not encapsulate
  the worst-case
• Instrumentation required to collect data
• Probe effect disrupts the temporal behaviour

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What we propose

• Combination of static analysis and testing
  techniques
• Testing measures the real behaviour of the
  processor
• Static analysis techniques dictate how these data
  are combined to produce a final estimate

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Instr1
Instr2 Instr3
Instr4 Instr5 Instr6 Instr7
Instr8 Instr9
Instr10
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Issues in Measurement-Based Analysis

• Well instrumented programs
• Instrumentation under constraints
• Generating test-data for temporal correctness
• Adequacy criteria for WCET coverage
• Probability that estimates are correct

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Conclusion

• Timeliness of many embedded systems relies on
  WCET analysis
• Static analysis struggles with advanced processor
  features
• Measurement-Based approach aims to combine static
  and testing techniques
• Accuracy the key as wider-issue processors become
  more prevalent