Hardware Reference
In-Depth Information
In the rest of this chapter, the following types of overload conditions will be analyzed:
Transient overloads due to aperiodic jobs
. This type of overload is typical of
event-triggered systems consisting of many aperiodic jobs activated by external
events. If the operating system is not designed to cope with excessive event ar-
rivals, the effects of an overload can be unpredictable and cause serious problems
on the controlled system. Experiments carried out by Locke [Loc86] have shown
that EDF can rapidly degrade its performance during overload intervals, and there
are cases in which the arrival of a new task can cause all the previous tasks to miss
their deadlines. Such an undesirable phenomenon, called the
Domino Effect
,is
depicted in Figure 9.3. Figure 9.3a shows a feasible schedule of a task set exe-
cuted under EDF. However, if at time
t
0
task
J
0
is executed, all the previous tasks
miss their deadlines (see Figure 9.3b).
J
1
J
2
J
3
J
4
(a)
J
0
J
1
J
2
J
3
J
4
t
0
(b)
Figure 9.3
Feasible schedule with Earliest Deadline First, in normal load condition (a).
Overload with Domino Effect due to the arrival of task
J
0
(b).
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