3Com WX3000 Series Operation Manual page 373

Figure 1-8 Diagram for WRR queuing
Packets to be sent
through this port
Packet
classification
WRR queue-scheduling algorithm schedules all the queues in turn and every queue can be assured of
a certain service time. Assume there are eight priority queues on a port. WRR configures a weight value
for each queue, which is w7, w6, w5, w4, w3, w2, w1, and w0. The weight value indicates the proportion
of obtaining resources. On a 100 M 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 5 Mbps 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.
3) SDWRR
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 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 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 the
Table 1-7.
Queue 1 Weight 1
Queue 2 Weight 2
Queue 2 weight 2
......
Queue N-1 Weight N-1
Queue N - 1 weight N -1
Queue N Weight N
Queue N weight N
1-12
Sent packets
Interface
Queue Sending queue
scheduling