3Com 4200G 12-Port Configuration Manual page 234

220 C 27: Q S C
HAPTER O ONFIGURATION
The disadvantage of SP queue is that: if there are packets in the queues with higher
priority for a long time in congestion, the packets in the queues with lower priority
will be "starved to death" because they are not served.
2 WRR queue
Figure 70 Diagram for WRR
Packets sent via this interface Packets sent via this interface
3 WRR queue-scheduling algorithm schedules all the queues in turn and every queue
can be assured of a certain service time. Assume there are 8 priority queues on the
port. WRR configures a weight value for each queue, which are w7, w6, w5, w4, w3,
w2, w1, and w0. The weight value indicates the proportion of obtaining resources.
On a 100M port, configure the weight value of WRR queue-scheduling algorithm to
50, 50, 30, 30, 10, 10, 10 and 10 (corresponding to w7, w6, w5, w4, w3, w2, w1,
and w0 in order). In this way, the queue with the lowest priority can get 5Mbps
bandwidth at least, and the disadvantage of SP queue-scheduling that the packets in
queues with lower priority may not get service for a long time is avoided. Another
advantage of WRR queue is that: though the queues are scheduled in order, the
service time for each queue is not fixed, that is to say, if a queue is empty, the next
queue will be scheduled. In this way, the bandwidth resources are made full use of.
SDWRR queue
Comparing with WRR queue, SDWRR queue further optimizes the delay and variation
for different queues.
For example, configure the weight value of queue0 and queue1 to 5 and 3
respectively. The processing procedures of WRR and SDWRR are as follows:
WRR: The packets whose weight value is 3 in queue1 are scheduled only after the
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packets whose weight value is 5 in the queue0 are scheduled. If there is a wide
difference between the weight values of two queues, the queue with high weight
value will cause great delay and variation for the queue with low weight value.
SDWRR: Two queues are scheduled in turn. Packets whose weight value is 1 in
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queue0 are scheduled first, and then packets whose weight value is 1 in queue1
are scheduled. The procedure is repeated until the scheduling for one queue is
over, and then SDWRR will schedule packets with the left weight values in the
other queue. The detailed scheduling sequence is described in Table 187.
queue1 weight1 queue1 weight1
queue2 weight2 queue2 weight2
...... ......
queueN - 1 weightN - 1 queueN - 1 weightN - 1
Classify Classify
queueN weightN queueN weightN
Packets sent Packets sent
Sending queue Sending queue
Dequeue Dequeue