Hardware Reference
In-Depth Information
normal execution
critical section
τ
1
τ
2
0
2 4
6
8 0
12
4 6
18
0 2 4
(a)
deadline miss
τ
1
τ
2
0
2
4
6
8
10
12
14
16
18
20
22
24
(b)
Figure 2.24
Scheduling anomaly in the presence of resource constraints: task
τ
1
meets its
deadline when the processor is executing at a certain speed
S
1
(a), but misses its deadline
when the speed is doubled (b).
task set is executed with double speed
S
2
=2
S
1
,
τ
1
misses its deadline, as clearly
illustrated in Figure 2.25b. This happens because, when
τ
1
arrives,
τ
3
already started
its execution and cannot be preempted (due to the non-preemptive mode).
It is worth observing that a set of non-preemptive tasks can be considered as a special
case of a set of tasks sharing a single resource (the processor) for their entire execution.
According to this view, each task executes as it were inside a big critical section with
a length equal to the task computation time. Once a task starts executing, it behaves as
it were locking a common semaphore, thus preventing all the other tasks from taking
the processor.
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