Hardware Reference
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τ 1
τ 2
τ 3
0
3
6
9
12
15
18
(a)
deadline miss
τ 1
τ 2
τ 3
0
3
6
9
12
15
18
(b)
Figure 2.25
Scheduling anomaly in the presence of non-preemptive tasks: task τ 1
meets
its deadline when the processor is executing at speed S 1
(a), but misses its deadline when
the speed is doubled (b).
ANOMALIES USING A DELAY PRIMITIVE
Another timing anomaly can occur when tasks using shared resources explicitly sus-
pend themselves through a delay ( T ) system call, which suspends the execution of the
calling task for T units of time. Figure 2.26a shows a case in which τ 1 is feasible and
has a slack time of 6 units when running at the highest priority, suggesting that it could
easily tolerate a delay of two units. However, if τ 1 executes a delay (2) at time t =2,
it gives the opportunity to τ 2 to lock the shared resource. Hence, when τ 1 resumes,
it has to block on the semaphore for 7 units, thus missing its deadline, as shown in
Figure 2.26b.
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