Limited-Preemption Scheduling of Sporadic Tasks Systems

Real-Time Systems Laboratory

RETIS Lab

Marko Bertogna

Research Area: Real-Time Scheduling and Resource Management

Introduction

• Sporadic task system with arbitrary deadlines = 1, 2,…, n with i = (ei ,di ,pi)

• Preemptive EDF is an optimal scheduler• Exact feasibility test:

with

for each , until a pseudo-polynomially

far point

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

Regular

Regular

min[cL,Q(DL - t)]

Non-Preemptive

JH

JL

t DL

Non-preemption function Q(t)

• Compute Q(t) such thatFeasibility 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

Same operations as in the EDF feasibility

check:

Computing Q(t)

1.

2. For every deadline D2, D3, …, Dm ≡ dmax :

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

Marko BertognaPhD student

marko@sssup.it

Real-Time Systems Laboratory

RETIS Lab

Thank you