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CprE 458/558: Real-Time Systems. Some practical issues. Finding WCET. Problem: Given a program and an architecture, find the worst-case execution time (WCET) of that program on the given architecture.

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Cpre 458 558 real time systems l.jpg

CprE 458/558: Real-Time Systems

Some practical issues

CprE 458/558: Real-Time Systems (G. Manimaran)


Finding wcet l.jpg
Finding WCET

Problem: Given a program and an architecture, find the worst-case execution time (WCET) of that program on the given architecture.

Fundamental limitation: Given an arbitrary program, the problem of deciding if it will ever stop is un-decidable. This is known Halting Problem (Turing machine).

By restricting the programming constructs used, one can get an upper bound in many cases.

CprE 458/558: Real-Time Systems (G. Manimaran)


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WCET Estimation Approaches

  • Program and Architecture analysis

    • Easy when architecture is simple – no cache, no pipeline, etc.

    • Difficult in modern architectures because of deep pipeline, multi-level cache, super-scalar feature, speculative execution and branch prediction.

  • Profiling based statistical approaches

    • Make several profiling runs of the program, get their exec. time to get the estimate of WCET.

    • Based on experimental results, estimate the confidence associated with some estimate of WCET.

CprE 458/558: Real-Time Systems (G. Manimaran)


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Practical considerations

  • Overruns: ACET > WCET

    • Abort the overrun job and perform exception handling

    • Value-based scheduling (drop less critical tasks)

    • Rescheduling, if possible

CprE 458/558: Real-Time Systems (G. Manimaran)


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Practical consideration – contd.

  • Mode change

    • Mission has multiple phases of execution

    • Each phase may have a separate set of periodic/aperiodic tasks

    • Smooth transition from one phase to another

      • Deleting tasks from old phase

      • Initiating tasks from new phase

      • Executing tasks from old and new phases during transition phase

    • Schedulability analysis must take into account multiple phases

    • Eg: flight control system: take-off, cruise, landing.

CprE 458/558: Real-Time Systems (G. Manimaran)


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