Transcript Best effort scheduling
CprE 458/558: Real-Time Systems
Best Effort Scheduling CprE 458/558: Real-Time Systems (G. Manimaran) 1
Best-Effort Scheduler
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No schedulability check Schedule construction – online
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Overload handling (handling timing faults)
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Value based scheduling Imprecise computation (m,k)-firm task scheduling
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Value based scheduling
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Task Ti :
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If Ti finishes by di, it offers a value of Vi. Else, it offers a value of 0 (sometimes a negative value).
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Best-Effort Scheduler (Contd.)
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Deadline scheduler (eg., EDF) – good for under/normal load
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Value-based scheduler (e.g., HVDF: Highest Value Density First) – good for overload
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Hybrid (Adaptive) scheduler --- good for all loads
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Heuristics Hi = function(value, deadline). Several heuristics exist.
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HVDF – Highest Value Density First
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Value density = Vi/Ci (i.e., value per unit computation time).
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Higher the value density, higher the importance and hence higher the priority.
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HDVF scheduler schedules tasks based on “value density”
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Competitive Analysis of BE scheduler
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The competitive factor, B A , of an on-line scheduling algorithm is defined as
V A
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S
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V CA
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S
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B A
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Where S: a given task set
for all S
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.
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Competitive Analysis of BE scheduler (contd.) •
The upper bound on the competitive factor for any on-line scheduling is
( 1 1 2 )
Where Y = highest value density / lowest value density
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When Y = 1 (i.e., Vi = Ci), the competitive factor is 0.25 (for single processor, same as the result discussed in chapter 2)
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