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)
delay(2)
blocked
deadline miss
τ 1
τ 2
0
2
4
6
8
10
12
14
16
18
20
22
24
(b)
Figure 2.26
has a slack of 6 units of
time when running at the highest priority (a), but cannot tolerate a self suspension of two
units (b).
Scheduling anomaly in the presence of a delay:
τ 1
The example shown in Figure 2.26 illustrates an anomaly in which a task with a large
slack cannot tolerate a self suspension of a much smaller value. Figure 2.27 shows
another case in which the suspension of a task can also cause a longer delay in a
different task, even without sharing any resource. When τ 1 is assigned a higher priority
than τ 2 , the resulting schedule shown in Figure 2.27a is feasible, with a slack for τ 1 of
3 units of time. However, if the third instance of τ 1 executes a delay (1) after one unit
of execution, τ 2 will miss its deadline.
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