FIXED-PRIORITY SERVERS - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 119

5

FIXED-PRIORITY SERVERS

5.1 INTRODUCTION

The scheduling algorithms treated in the previous chapters deal with homogeneous
sets of tasks, where all computational activities are either aperiodic or periodic. Many
real-time control applications, however, require both types of processes, which may
also differ for their criticality. Typically, periodic tasks are time-driven and execute
critical control activities with hard timing constraints aimed at guaranteeing regular
activation rates. Aperiodic tasks are usually event-driven and may have hard, soft, or
non-real-time requirements depending on the specific application.

When dealing with hybrid task sets, the main objective of the kernel is to guarantee the
schedulability of all critical tasks in worst-case conditions and provide good average
response times for soft and non-real-time activities. Off-line guarantee of event-driven
aperiodic tasks with critical timing constraints can be done only by making proper
assumptions on the environment; that is, by assuming a maximum arrival rate for
each critical event. This implies that aperiodic tasks associated with critical events are
characterized by a minimum interarrival time between consecutive instances, which
bounds the aperiodic load. Aperiodic tasks characterized by a minimum interarrival
time are called sporadic . They are guaranteed under peak-load situations by assuming
their maximum arrival rate.

If the maximum arrival rate of some event cannot be bounded a priori, the associated
aperiodic task cannot be guaranteed off-line, although an online guarantee of individ-
ual aperiodic requests can still be done. Aperiodic tasks requiring online guarantee
on individual instances are called firm . Whenever a firm aperiodic request enters the
system, an acceptance test can be executed by the kernel to verify whether the request