Page 93 - DCAP608_REAL TIME SYSTEMS

Real Time Systems Sandeep Kumar, Lovely Professional University

Notes Unit 9: Concept of Clock-driven Scheduling

CONTENTS
Objectives
Introduction

9.1 Notations and Assumptions
9.2 Static, Timer-Driven Schedules
9.3 General Structure of Cyclic Schedules

9.4 Cyclic Executives
9.5 Summary
9.6 Keywords
9.7 Review Questions
9.8 Further Readings

Objectives

After studying this unit, you will be able to:
 Describe Notations and Assumptions

 Enumerate Static, Timer-Driven Schedules
 Explain General Structure of Cyclic Schedules
 Analyse Cyclic Executives

Introduction

The clock-driven schedulers are those in which the scheduling points are determined by the
interrupts received from a clock. In the event-driven ones, the scheduling points are defined by
certain events which precludes clock interrupts. Clock driven scheduling has an ability to consider
complex dependencies, communication delays, and resource contention among jobs when
constructing the static schedule, guaranteeing absence of deadlocks and unpredictable delays.
Entire schedule is captured in a static table. Different operating modes can be represented by
different tables. No concurrency control or synchronization required. If completion time jitter
requirements exist, it can be captured in the schedule. In this unit, we will discuss the concept of
clock driven system.

9.1 Notations and Assumptions

Clock-driven approach is appropriate only when the system is deterministic, excluding a few
aperiodic and sporadic jobs. The restricted assumptions are:
 There are n periodic tasks in the system; n is fixed in an operation mode.
 The parameters of all periodic tasks are known a priori. Variations in the inter release
times of job in any periodic task are negligibly small i.e. each job in T is released p units
i i
of time after the previous job in T .
i

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