Administrator's Guide v4.1.1 Manual

FIGURE 54 WRR schedule — two queues
Deficit Weighted Round Robin (DWRR) is an improved version of WRR. DWRR remembers the
excess used when a queue goes over its bandwidth allocation and reduces the queue’s bandwidth
allocation in the subsequent rounds. This way the actual bandwidth usage is closer to the defined level
when compared to WRR.
Traffic class scheduling policy
Traffic classes are numbered from 0 to 7, with higher-numbered traffic classes treated as having a
higher priority. Brocade switches provide full flexibility in controlling the number of SP-to-WRR queues.
The number of SP queues is specified as SP1 through 8, then the highest-priority traffic classes are
configured for SP service and the remaining eight are WRR serviced. The supported scheduling
configurations listed in the table below describes the set of scheduling configurations supported.
When you configure the QoS queue to use strict priority 4 (SP4), then traffic class 7 will use SP4,
traffic class 6 will use SP3, and so on down the list. You use the strict priority mappings to control how
the different traffic classes will be routed in the queue.
Supported scheduling configurations TABLE 77
Traffic Class SP0 SP1 SP2 SP3 SP4 SP5 SP6 SP8
7 WRR8 SP1 SP2 SP3 SP4 SP5 SP6 SP8
6 WRR7 WRR7 SP1 SP2 SP3 SP4 SP5 SP7
5 WRR6 WRR6 WRR6 SP1 SP2 SP3 SP4 SP6
4 WRR5 WRR5 WRR5 WRR5 SP1 SP2 SP3 SP5
3 WRR4 WRR4 WRR4 WRR4 WRR4 SP1 SP2 SP4
2 WRR3 WRR3 WRR3 WRR3 WRR3 WRR3 SP1 SP3
1 WRR2 WRR2 WRR2 WRR2 WRR2 WRR2 WRR2 SP2
0 WRR1 WRR1 WRR1 WRR1 WRR1 WRR1 WRR1 SP1
The figure below shows that extending the frame scheduler to a hybrid SP+WRR system is fairly
straightforward. All SP queues are considered strictly higher priority than WRR so they are serviced
first. Once all SP queues are drained, then the normal WRR scheduling behavior is applied to the non-
empty WRR queues.
Traffic class scheduling policy
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