CprE 458/558: Real-Time Systems

1
CprE 458/558 Real-Time Systems

• Best Effort Scheduling

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 ltCi, Pi, Vigt 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).

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.

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

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.

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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)