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CprE 458/558: Real-Time Systems

CprE 458/558: Real-Time Systems. Best Effort Scheduling. Best-Effort Scheduler. No schedulability check Schedule construction – online Overload handling (handling timing faults) Value based scheduling Task Ti : <Ci, Pi, Vi> where Vi is the value offered by Ti.

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CprE 458/558: Real-Time Systems

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  1. CprE 458/558: Real-Time Systems Best Effort Scheduling CprE 458/558: Real-Time Systems (G. Manimaran)

  2. Best-Effort Scheduler • No schedulability check • Schedule construction – online • Overload handling (handling timing faults) • Value based scheduling • Task Ti : <Ci, Pi, Vi> where Vi is the value offered by Ti. • If Ti finishes before di, it offers a value of Vi, Else it offers a value of 0 (sometimes a negative value). • Types of Schedulers • Deadline scheduler (eg., EDF) – good for under/normal load • Value-based scheduler (e.g., HVDF) – good for overload • Hybrid (Adaptive) scheduler --- good for all loads • Hi = f(EDF, HVDF). • Several heuristics exist. CprE 458/558: Real-Time Systems (G. Manimaran)

  3. HVDF – Highest Value Density First • Value density = Vi/Ci (ie., value per unit computation time). • Higher the value density, higher the importance and hence higher the priority. • HDVF scheduler schedules tasks based on “value density” CprE 458/558: Real-Time Systems (G. Manimaran)

  4. Competitive Analysis of BE scheduler • The competitive factor, , of an on-line scheduling algorithm is defined as • The upper bound on the competitive factor for the on-line scheduling is Where Y = highest value density / lowest value density • When Y = 1 (ie., Vi = Ci), the competitive factor is 0.25 (for single processor, same as the result discussed beginning chapter 2) CprE 458/558: Real-Time Systems (G. Manimaran)

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