Page 120 - DCAP608_REAL TIME SYSTEMS

Gurwinder Kaur, Lovely Professional University Unit 12: Priority-driven Scheduling of Periodic Tasks

Unit 12: Priority-driven Scheduling of Periodic Tasks Notes

CONTENTS
Objectives
Introduction

12.1 Static Assumptions
12.2 Fixed-Priority versus Dynamic-Priority Algorithms
12.2.1 Rate Monotonic and Deadline Monotonic Algorithms

12.2.2 Some Well Known Dynamic Algorithms
12.3 Maximum Schedulable Utilization
12.4 Summary
12.5 Keywords
12.6 Review Questions

12.7 Further Readings

Objectives

After studying this unit, you will be able to:

 Describe Static Assumptions
 Enumerate fixed-Priority Versus Dynamic-Priority Algorithms
 Explain Maximum Schedulable Utilization

Introduction

Priority-driven scheduling is easy to implement. It does not require the information on the
release times and execution times of the jobs a priori. The run-time overhead due to maintaining
a priority queue of ready jobs can be made small. Priority-driven algorithms differ from each
other in how priorities are assigned to jobs. We classify algorithms for scheduling periodic tasks
into two types, that is, Fixed-priority and Dynamic-priority. A fixed-priority algorithm assigns
the same priority to all the jobs in each task. A dynamic-priority algorithm assigns different
priorities to the individual jobs in each task. Most real-time scheduling algorithms of practical
interest assign fixed priority to individual jobs.

12.1 Static Assumptions

Consider the assumptions given below:
 The tasks are independent

 There are no aperiodic and sporadic tasks
 Job is ready for execution as soon as it is released
 Jobs are preemptible
 Scheduling decisions are made immediately upon job release and completion

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