Hardware Reference
In-Depth Information
τ
1
0
8
16
24
τ
2
0
12
24
2
2
2
1
aperiodic
requests
0
3
6
9
12
14
24
+3
+2
+1
+1
3
DSS
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Figure 6.4
Dynamic Sporadic Server example.
Figure 6.4 illustrates an EDF schedule obtained on a task set consisting of two periodic
tasks with periods
T
1
=8,
T
2
=12and execution times
C
1
=2,
C
2
=3, and a DSS
with period
T
s
=6and capacity
C
s
=3.
At time
t
=0, the server capacity is initialized at its full value
C
s
=3. Since
there are no aperiodic requests pending, the processor is assigned to task
τ
1
, which
has the earliest deadline. At time
t
=3, an aperiodic request with execution time 2
arrives and, since
C
s
>
0, the first replenishment time and the server deadline are
set to
RT
1
=
d
s
=3+
T
s
=9. Being
d
s
the earliest deadline, DSS becomes the
highest-priority task in the system and the request is serviced until completion. At
time
t
=5, the request is completed and no other aperiodic requests are pending;
hence, a replenishment of two units of time is scheduled to occur at time
RT
1
=9.
At time
t
=6, a second aperiodic requests arrives. Being
C
s
>
0, the next replenish-
ment time and the new server deadline are set to
RT
2
=
d
s
=6+
T
s
=12. Again,
the server becomes the highest-priority task in the system (we assume that ties among
tasks are always resolved in favor of the server) and the request receives immediate
service. This time, however, the capacity has only one unit of time available, and it
gets exhausted at time
t
=7. Consequently, a replenishment of one unit of time is
scheduled for
RT
2
=12, and the aperiodic request is delayed until
t
=9, when
C
s
becomes again greater than zero. At time
t
=9, the next replenishment time and the
new deadline of the server are set to
RT
3
=
d
s
=9+
T
s
=15. As before, DSS be-
comes the highest-priority task; thus, the aperiodic request receives immediate service
and finishes at time
t
=10. A replenishment of one unit is then scheduled to occur at
time
RT
3
=15.
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