Page 109 - DCAP608_REAL TIME SYSTEMS

Real Time Systems Sandeep Kumar, Lovely Professional University

Notes Unit 11: Application of Clock-driven Scheduling

CONTENTS
Objectives
Introduction

11.1 Practical Considerations and Generalizations
11.1.1 Frame overruns
11.1.2 Mode Changes

11.1.3 Multi-Processors
11.2 Algorithm for Constructing Static Schedules
11.3 Pros and Cons of Clock-Driven Scheduling
11.4 Summary
11.5 Keywords

11.6 Review Questions
11.7 Further Readings

Objectives

After studying this unit, you will be able to:

 Describe Practical Considerations and Generalizations
 Enumerate Algorithm for Constructing Static Schedules
 Analyse Pros and Cons of Clock-Driven Scheduling

Introduction

We can schedule aperiodic jobs in the background after completing the scheduling of periodic
jobs in each frame. Otherwise, the average response time of aperiodic jobs can be improved.
This can be done by scheduling them in the slack in every frame. Likewise, we schedule sporadic
jobs during the slack times not utilized by periodic job slices in the frames. When a sporadic job
is accepted, it is scheduled between all sporadic jobs in the EDF order during the time intervals
not utilized by periodic jobs slices. In this unit, we will discuss some practical considerations of
clock driven scheduling. Also we will discuss the pros and cons of clock driven scheduling.

11.1 Practical Considerations and Generalizations

In this section, we will discuss various practical considerations.
11.1.1 Frame Overruns

A frame overrun can occur for many reasons as follows:
 When the execution time of a job is input data dependent, it can become unexpectedly
large for some rare combination of input value which is not taken into account in the
precomputed schedule.

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