Hardware Reference
In-Depth Information
C
T i
i
Server
τ
1
4
1
C
= 2
s
τ
2
6
2
T
= 5
s
τ
1
τ 2
aperiodic
requests
2
1
2
1
0
2
4
6
8
10
12
14
16
18
20
22
24
C s
2
1
0
2
4
6
8
10
12
14
16
18
20
22
24
Figure 5.3
Example of a Polling Server scheduled by RM.
the periodic tasks, and, once active, it serves the aperiodic requests within the limit of
its budget. The ordering of aperiodic requests does not depend on the scheduling al-
gorithm used for periodic tasks, and it can be done by arrival time, computation time,
deadline, or any other parameter.
The Polling Server (PS) is an algorithm based on such an approach. At regular in-
tervals equal to the period T s , PS becomes active and serves the pending aperiodic
requests within the limit of its capacity C s . If no aperiodic requests are pending, PS
suspends itself until the beginning of its next period, and the budget originally allo-
cated for aperiodic service is discharged and given periodic tasks [LSS87, SSL89].
Note that if an aperiodic request arrives just after the server has suspended, it must
wait until the beginning of the next period, when the server capacity is replenished at
its full value.
Figure 5.3 illustrates an example of aperiodic service obtained through a Polling Server
scheduled by RM. The aperiodic requests are reported on the third row, whereas the
fourth row shows the server capacity as a function of time. Numbers beside the arrows
indicate the computation times associated with the requests.
In the example shown in Figure 5.3, the Polling Server has a period T s =5and a
capacity C s =2, so it runs with an intermediate priority with respect to the other
 
Search WWH ::




Custom Search