RETIS Lab RealTime Systems Laboratory Research Area RealTime
RETIS Lab Real-Time Systems Laboratory Research Area: Real-Time Scheduling and Resource Management Limited-Preemption Scheduling of Sporadic Tasks Systems Marko Bertogna
Introduction • Sporadic task system with arbitrary deadlines t = t 1, t 2, …, tn with ti = (ei , di , pi) • Preemptive EDF is an optimal scheduler • Exact feasibility test: with for each far point , until a pseudo-polynomially
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
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 JL Regular min[c. L, Q(DL - t)] Non-Preemptive DL
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
Computing Q(t) 1. 2. For every deadline D 2, D 3, …, Dm ≡ dmax : Same operations as in the EDF feasibility check:
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
Simulations • uniform Ui • n=5 • pi in [10, 1000] • t in [0, 106]
Simulations • uniform Ui • n = 10 • pi in [10, 1000] • t in [0, 106]
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
RETIS Lab Real-Time Systems Laboratory Thank you Marko Bertogna Ph. D student marko@sssup. it
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