Page 86 - DCAP608_REAL TIME SYSTEMS

Sandeep Kumar, Lovely Professional University Unit 8: Working of Real-time Scheduling

Unit 8: Working of Real-time Scheduling Notes

CONTENTS
Objectives
Introduction

8.1 Challenges in Validating Timing Constraint in Priority Driven System
8.2 Off-line Versus On-line Scheduling
8.3 Summary

8.4 Keywords
8.5 Review Questions
8.6 Further Readings

Objectives

After studying this unit, you will be able to:

 Give an overview of multiprocessor and distributed real-time systems
 Describe Challenges in Validating Timing Constraint in Priority Driven System
 Analyse Off-line Versus On-line Scheduling.

Introduction

In multiprocessor and distributed real-time systems, scheduling jobs dynamically on processors
can be used to achieve better performance. However, analytical and efficient validation methods
for determining whether all the timing constraints are met do not yet exist for systems using
modern dynamic scheduling strategies, and exhaustive methods are often infeasible or unreliable
since the execution time and release time of each job may vary.

8.1 Challenges in Validating Timing Constraint in Priority

Driven System

Priority-driven system or schedulers are online schedulers which comprise of following features:

 Schedule is not pre-computed.
 Scheduler assigns priority to released jobs and places them in ready job queue in priority
order.

 At each scheduling decision time, scheduler updates the ready job queue and schedules job
with highest priority.
The assumptions of Priority-Driven Scheduling are:

 Every job is ready for execution as soon as it is released, and can be preempted at any time.
 Scheduling decisions are made immediately upon job releases and completions.
 The context switch overhead is negligibly small compared with execution times.
 The number of priority levels is unlimited.

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