Brocade Communications Systems 53-1001761-01 Deficit weighted round robin scheduling, Scheduling

Scheduling 9

Figure 10 describes the frame scheduling order for an SP scheduler servicing two SP queues. The higher numbered queue, SP2, has a higher priority.

FIGURE 10 Strict priority schedule — two queues

Deficit weighted round robin scheduling

Weighted Round Robin (WRR) scheduling is used to facilitate controlled sharing of the network bandwidth. WRR assigns a weight to each queue; that value is then used to determine the amount of bandwidth allocated to the queue. The round robin aspect of the scheduling allows each queue to be serviced in a set ordering, sending a limited amount of data before moving onto the next queue and cycling back to the highest priority queue after the lowest priority is serviced.

Figure 11 describes the frame scheduling order for a WRR scheduler servicing two WRR queues. The higher numbered queue is considered higher priority (WRR2) and the weights indicate the network bandwidth should be allocated in a 2:1 ratio between the two queues. In Figure 11 WRR2 should receive 66 percent of bandwidth and WRR1 receives 33 percent. The WRR scheduler tracks the extra bandwidth used and subtracts it from the bandwidth allocation for the next cycle through the queues. In this way, the bandwidth utilization statistically matches the queue weights over longer time periods.

FIGURE 11 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

The traffic classes are numbered from 0 to 7; higher numbered traffic classes are considered higher priority. The Brocade 8000 provides full flexibility in controlling the number of SP-to-WRR queues. The number of SP queues is specified in N (SP1 through 8), then the highest priority traffic classes are configured for SP service and the remaining eight are WRR serviced. Table 19 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.

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