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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 (chapter 2) Imprecise computation (chapter 4) (m,k)-firm task scheduling (chapter 4)

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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 (chapter 2) • Imprecise computation (chapter 4) • (m,k)-firm task scheduling (chapter 4) • Value based scheduling • Task Ti : <Ci, Pi, Vi> where Vi is the value offered by Ti. • If Ti finishes by di, it offers a value of Vi. Else, it offers a value of 0 (sometimes a negative value). CprE 458/558: Real-Time Systems (G. Manimaran)

  3. Best-Effort Scheduler (Contd.) • Deadline scheduler (eg., EDF) – good for under/normal load • Value-based scheduler (e.g., HVDF: Highest Value Density First) – good for overload • Hybrid (Adaptive) scheduler --- good for all loads • Heuristics Hi = function(value, deadline). • Several heuristics exist. CprE 458/558: Real-Time Systems (G. Manimaran)

  4. HVDF – Highest Value Density First • Value density = Vi/Ci (i.e., 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)

  5. Competitive Analysis of BE scheduler • The competitive factor, BA , of an on-line scheduling algorithm is defined as Where S: a given task set VA(S): value produced by given scheduler A VCA(S): value produced by clairvoyant scheduler, the scheduler which knows complete knowledge of all tasks at the beginning itself. CprE 458/558: Real-Time Systems (G. Manimaran)

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

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