FIXED-PRIORITY SERVERS - Hard Real-Time Computing Systems: Predictable Scheduling Algorithms and Applications - page 120

can be served within its deadline. If such a guarantee cannot be done, the request is
rejected.

In the next sections, we present a number of scheduling algorithms for handling hybrid
task sets consisting of a subset of hard periodic tasks and a subset of soft aperiodic
tasks. All algorithms presented in this chapter rely on the following assumptions:

Periodic tasks are scheduled based on a fixed-priority assignment; namely, the
Rate-Monotonic (RM) algorithm;

All periodic tasks start simultaneously at time t =0and their relative deadlines
are equal to their periods.

Arrival times of aperiodic requests are unknown.

When not explicitly specified, the minimum interarrival time of a sporadic task is
assumed to be equal to its deadline.

All tasks are fully preemptable.

Aperiodic scheduling under dynamic priority assignment is discussed in the next chap-
ter.

5.2 BACKGROUND SCHEDULING

The simplest method to handle a set of soft aperiodic activities in the presence of
periodic tasks is to schedule them in background; that is, when there are not periodic
instances ready to execute. The major problem with this technique is that, for high
periodic loads, the response time of aperiodic requests can be too long for certain
applications. For this reason, background scheduling can be adopted only when the
aperiodic activities do not have stringent timing constraints and the periodic load is
not high.

Figure 5.1 illustrates an example in which two periodic tasks are scheduled by RM,
while two aperiodic tasks are executed in background. Since the processor utiliza-
tion factor of the periodic task set ( U =0 . 73) is less than the least upper bound for
two tasks ( U lub (2) 0 . 83), the periodic tasks are schedulable by RM. Note that the

guarantee test does not change in the presence of aperiodic requests, since background
scheduling does not influence the execution of periodic tasks.