CprE 458/558: Real-Time Systems (G. Manimaran)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 –Imprecise computation –(m,k)-firm task scheduling Value based scheduling –Task Ti : 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, B A, 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)