Hardware Reference
In-Depth Information
τ
1
τ
2
aperiodic
requests
1
2
0
2
4
6
8
10
12
14
16
18
20
22
24
Figure 5.1
Example of background scheduling of aperiodic requests under Rate Mono-
tonic.
RM
Periodic Tasks
High-Priority Queue
CPU
FCFS
Aperiodic Tasks
Low-Priority Queue
Figure 5.2
Scheduling queues required for background scheduling.
The major advantage of background scheduling is its simplicity. As shown in Fig-
ure 5.2, two queues are needed to implement the scheduling mechanism: one (with
a higher priority) dedicated to periodic tasks and the other (with a lower priority) re-
served for aperiodic requests. The two queueing strategies are independent and can be
realized by different algorithms, such as RM for periodic tasks and First Come First
Served (FCFS) for aperiodic requests. Tasks are taken from the aperiodic queue only
when the periodic queue is empty. The activation of a new periodic instance causes
any aperiodic tasks to be immediately preempted.
5.3
POLLING SERVER
The average response time of aperiodic tasks can be improved with respect to back-
ground scheduling through the use of a
server
; that is, a periodic task whose purpose
is to service aperiodic requests as soon as possible. Like any periodic task, a server
is characterized by a period
T
s
and a computation time
C
s
, called server
capacity
,or
server
budget
. In general, the server is scheduled with the same algorithm used for
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