Hardware Reference
In-Depth Information
EXAMPLE
Consider the periodic task set shown in Table 9.1. The equivalent utilization factor
of the periodic task set is
U
skip
p
=4
/
5, while
U
max
s
=0
.
27, leaving a bandwidth of
U
ski
p
=1
/
5 for the aperiodic tasks. Three aperiodic jobs
J
1
,
J
2
, and
J
3
are released at times
t
1
=0,
t
2
U
s
=1
−
=6, and
t
3
=18; moreover, they have computation
times
C
ape
1
=1,
C
ape
2
=2, and
C
ape
3
=1, respectively.
Task
Task1
Task2
Computation
2
2
Period
3
5
∞
Skip Parameter
2
U
p
1.07
U
skip
p
0.8
U
skip
p
1
−
0.2
U
max
s
0.27
Table 9.1
A schedulable task set.
skip
skip
skip
skip
τ
1
0
3
6
9
12
15
18
21
24
27
τ
2
0
5
10
15
20
25
CBS
U
s
=1
/
5
0
5
6
11
16
18
23
Figure 9.26
Schedule produced by EDF+CBS for the task set shown in Table 9.1.
Supposing aperiodic activities are scheduled by a CBS server with budget
Q
s
=1
and period
T
s
=5, Figure 9.26 shows the resulting schedule under EDF+CBS. Note
that
J
2
has a deadline postponement (according to CBS rules) at time
t
=10with
new server deadline
d
new
=
d
old
+
T
s
=11+5=16. According to the sufficient
schedulability test provided by Theorem 9.7, the task set is schedulable when the CBS
is assigned a bandwidth
U
s
U
ski
p
. However, this task set is also schedulable
with a bandwidth
U
s
=0
.
25, greater than 1
=1
−
U
skip
p
but less than
U
max
s
−
, although this
is not generally true.
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