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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

CprE 458/558: Real-Time Systems

Some practical issues

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

finding wcet
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)

wcet estimation approaches
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)

practical considerations
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)

practical consideration contd
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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