Page 65 - DCAP608_REAL TIME SYSTEMS

Real Time Systems Sandeep Kumar, Lovely Professional University

Notes Unit 6: Commonly used Approaches to
Real-time Scheduling

CONTENTS
Objectives
Introduction

6.1 Clock Driven Approach
6.2 Weight Round–Robin Approach
6.3 Priority Driven Approach
6.4 Dynamic versus Static System

6.5 Summary
6.6 Keywords
6.7 Review Questions
6.8 Further Readings

Objectives

After studying this unit, you will be able to:
 Describe Clock Driven Approach
 Enumerate Weight Round–Robin Approach
 Explain Priority Driven Approach

 Analyse Dynamic versus Static System

Introduction

Commonly used Approaches to Real-time Scheduling are clock driven approach, weight round
robin approach, priority-driven approach, etc. Round-robin is commonly used for scheduling
time-shared applications. Weighted Round-Robin extends the basic round-robin algorithm with
weights. Priority-Driven Scheduling assigns priorities to jobs. Overview of different classes of
real-time scheduling algorithms is given in this unit.

6.1 Clock Driven Approach

In clock-driven scheduling, decisions on what jobs execute at what times are made at specific
time instants. Instants are chosen a priori before the system begins execution. Usually all the
parameters of hard real-time jobs are fixed and known.

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Caution Schedule of the jobs is computed off-line and is stored for use at run time.
The scheduler schedules the jobs according to this schedule at each scheduling decision time thus
scheduling overhead during run-time is minimized. A hardware timer can be used; the timer is
set to expire periodically without the intervention of the scheduler.

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