CprE 458/558: Real-Time Systems (G. Manimaran)1 CprE 458/558: Real-Time Systems Some practical issues

CprE 458/558: Real-Time Systems (G. Manimaran)2 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)3 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)4 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)5 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.