LimitedPreemption Scheduling of Sporadic Tasks Systems

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Limited-Preemption Scheduling of Sporadic Tasks
Systems
RETIS Lab
Real-Time Systems Laboratory
Research Area Real-Time Scheduling and Resource
Management
Marko Bertogna
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Introduction

• Sporadic task system with arbitrary deadlines
• t t1, t2,, tn with ti (ei ,di ,pi)
• Preemptive EDF is an optimal scheduler
• Exact feasibility test
• with

for each , until a
pseudo-polynomially far point
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To preempt or not?

• PREEMPTIVE
• Optimal schedulability performances
• Need to use protocols for the access to shared
  resources

• NON-PREEMPTIVE
• Higher feasibility overhead
• Lower run-time overhead
• Simplified access to shared resources

Ideal situation optimal scheduling algorithm
with low run-time overhead
Allow preemption only when necessary for
maintaining feasibility
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Limited-preemption EDF

• Non-preemption function Q(t)
• Jobs priorities according to EDF
• Two modes regular and non-preemptive
• Initially, a job JL executes in regular mode
• When a higher priority job JH arrives, JL goes in
  non-preemptive mode

JH
Regular
t
mincL,Q(DL - t)
DL
JL
Regular
Non-Preemptive
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Non-preemption function Q(t)

• Compute Q(t) such that
• Feasibility is maintained
• Non-preemptive sections as large as possible
• Properties of Q(t)
• Monotonic non-increasing
• Changes value only at time-instants corresponding
  to task deadlines in a synchronous periodic
  release sequence

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Computing Q(t)

• For every deadline D2, D3, , Dm dmax

Same operations as in the EDF feasibility check
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Complexity

• Pseudo-polynomial complexity
• Comes for free when feasibility has to be checked
  as well
• When storing the Q(t) table, possible to discard
  some value, finding suboptimal results
• Very small memory requirements (from simulations)
• No more than 9 points of discontinuity
• Average number of 3 discontinuities

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Simulations

• uniform Ui
• n 5
• pi in 10,1000
• t in 0,106

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Simulations

• uniform Ui
• n 10
• pi in 10,1000
• t in 0,106

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Considerations and conclusions

• Optimal scheduling algorithm based on EDF
• Reduced number of context changes
• Small computational complexity and memory
  requirements
• Advantages w.r.t. preemptive EDF
• Lower run-time overhead
• Easy way to deal with shared resources
• Enhanced predictability

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Marko BertognaPhD studentmarko_at_sssup.it
RETIS Lab
Real-Time Systems Laboratory
Thank you
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Q(t) discontinuities