Brocade Communications Systems 53-1002163-02 manual Scheduling, Strict priority scheduling

Scheduling 11

3. Enter the copy command to save the running-config file to the startup-config file.

switch(config)#do copy running-config startup-config

Scheduling

Scheduling arbitrates among multiple queues waiting to transmit a frame. Fabric OS v7.0.0 supports both Strict Priority (SP) and Deficit Weighted Round Robin (DWRR) scheduling algorithms. Also supported is the flexible selection of the number of traffic classes using SP-to-DWRR. When there are multiple queues for the same traffic class, then scheduling takes these equal priority queues into consideration.

Strict priority scheduling

Strict priority scheduling is used to facilitate support for latency-sensitive traffic. A strict priority scheduler drains all frames queued in the highest priority queue before continuing on to service lower priority traffic classes. A danger with this type of service is that a queue can potentially starve out lower priority traffic classes.

Figure 8 describes the frame scheduling order for an SP scheduler servicing two SP queues. The higher numbered queue, SP2, has a higher priority.

FIGURE 8 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 9 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 9 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 9 WRR schedule — two queues

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