Hardware Reference
In-Depth Information
Transient overloads due to task overruns
. This type of overload can occur
both in event-triggered and time-triggered systems, and it is due to periodic or
aperiodic tasks that sporadically execute (or are activated) more than expected.
Under Rate Monotonic, an overrun in a task
τ
i
does not affect tasks with higher
priority, but any of the lower priority tasks could miss their deadline. Under EDF,
a task overrun can potentially affect all the other tasks in the system. Figure 9.4
shows an example of execution overrun in an EDF schedule.
τ
1
τ
2
τ
3
0
2
4
6
8
10
12
14
16
18
Figure 9.4
Effect of an execution overrun in an EDF schedule.
Permanent overloads in periodic task systems
. This type of overload occurs
when the total utilization factor of the periodic task set is greater than one. This
can happen either because the execution requirement of the task set was not cor-
rectly estimated, or some unexpected activation of new periodic tasks, or some
of the current tasks increased their activation rate to react to some change in the
environment. In such a situation, computational activities start accumulating in
the system's queues (which tend to become longer and longer, if the overload per-
sists), and tasks response times tend to increase indefinitely. Figure 9.5 shows the
effect of a permanent overload condition in a Rate Monotonic schedule, where
τ
2
misses its deadline and
τ
3
can never execute.
τ
1
τ
2
τ
3
0
2
4
6
8
10
12
14
16
18
Figure 9.5
Example of a permanent overload under Rate Monotonic:
τ
2
misses its dead-
line and
τ
3
can never execute.
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