Alcatel-Lucent 7210 SAS E OS Quality Of Service Manual page 49

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7210 SAS-E OS Quality of Service Guide
Displayed in Table 17, CoS queues 7 and 6 each have a minimum bandwidth specification
of 10 Mbps, whereas the remaining QoS queues have a minimum bandwidth specification
of 50 Mbps. All CoS queues have a maximum bandwidth specification of 1 Gbps. The
goal of these settings is to guarantee the minimum bandwidth settings for each of the
queues while also allowing each CoS queue to fully use the egress port capability by
having the maximum bandwidth setting at 1 Gbps. The strict priority scheduler mode
provides low latency service for CoS queues 6 and 7 while their minimum bandwidth
guarantees are being satisfied.
Round robin scheduling across CoS queues — The round robin scheduler mode provides
round robin arbitration across the CoS queues. The scheduler visits each backlogged CoS
queue, servicing a single packet at each queue before moving on to the next one. The
purpose of the round robin scheduler is to provide fair access to the egress port bandwidth
(at a packet level). This works best when packet sizes are approximately comparable.
Weighted round robin (WRR) — In WRR mode, the scheduler provides access to each
CoS queue in round robin order.When the scheduler is providing access to a particular
queue, it services a configurable number of back-to-back packets before moving on to the
subsequent CoS queue. A value of 0 is used to designate that a particular queue be
considered to be a part of a hybrid Strict + WRR configuration. The values 1 to 5 are used
to indicate the number of back-to-back packets to be serviced when the scheduler is
servicing a particular CoS queue. If the weight specified is N but if there are <N packets in
the queue, the scheduler continues working and moves on to the next backlogged queue.
Weighted deficit round robin (WDRR) scheduling— An inherent limitation of the WRR
mode is that bandwidth is allocated in terms of packets. WRR works well if the average
packet size for each CoS queue flow is known.WDRR aims at addressing this issue.
WDRR provides a bandwidth allocation scheduler mode that takes into account the
variably-sized packet issue by maintaining sufficient state information when arbitrating
across the CoS queues.
A weight value of 1 to 5 can be configured for each queue. Based on the weights provided
respective amount of bytes is de-queued from the queue. A value of 0 is used to designate
that a particular queue be considered to be a part of a hybrid Strict + WDRR configuration.
If a weight value of 1 is given for queue 1 and 5 is given for queue 2, then we will see
traffic out of the port in the ratio of 1:5 between the queues (1 and 2), provided no traffic is
flowing in the other queues. A weight value of 1 will actually pump out 2Kbytes from that
queue, a value of 5 will pump out 10 Kbytes. Twice of the weight value given will be
pumped out.
Strict + WRR/WDRR — If the WRR/WDRR weight associated with a particular CoS
queue is set to 0, the queue is considered to be in a strict priority mode. This indication
means that the scheduler treats the set of queues with a weight of 0 as strict priority
queues. This set of strict priority queues is serviced first in the order of their CoS
numbering (higher numbered CoS queue receives service before smaller numbered
queues).
QoS Policies
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