ZL50404
7.2.1
Strict Priority
Data Sheet
When strict priority is part of the scheduling algorithm, if a queue has any frame to transmit, it goes first. For RMAC
ports, this is an easy way to provide the different service. For all recognizable traffic, the bandwidth is guaranteed to
100% of the line rate. This scheme works as long as the overall high priority bandwidth is not over the line rate and
the latency on all the low priority traffic is don’t care. The strict priority queue in the MMAC and CPU ports is similar
to RMAC ports other than having 4 queues instead of 2 queues. The priority queue P0 can be scheduled only if the
priority queue P1 is empty, so as to priority queues P2 and P3. The lowest priority queue is treated as best effort
queue.
Because we do not provide any assurances for best effort traffic, we do not enforce latency by dropping best effort
traffic. Furthermore, because we assume that strict priority traffic is carefully controlled before entering the
ZL50404, we do not enforce a fair bandwidth partition by dropping strict priority traffic. To summarize, dropping to
enforce bandwidth or delay does not apply to strict priority or best effort queues. We only drop frames from best
effort and strict priority queues when queue size is too long or global / class buffer resources become scarce.
7.2.2
Weighted Fair Queuing
In some environments – for example, in an environment in which delay assurances are not required, but precise
bandwidth partitioning on small time scales is essential, WFQ may be preferable to a strict assurance scheduling
discipline. The ZL50404 provides this kind of scheduling algorithm on MMAC port only. The user sets four WFQ
“weights” such that all weights are whole numbers and sum to 64. This provides per-class bandwidth partitioning
with granular within 2%.
In WFQ mode, though we do not assure frame latency, the ZL50404 still retains a set of dropping rules that helps to
prevent congestion and trigger higher level protocol end-to-end flow control.
7.3
WRED Drop Threshold Management Support
To avoid congestion, the Weighted Random Early Detection (WRED) logic drops packets according to specified
parameters. The following table summarizes the behavior of the WRED logic.
Px > WRED_L1
High Drop
Low Drop
X%
Y%
Px > WRED_L2
100%
Z%
BM Reject
100%
100%
Table 8 - WRED Logic Behaviour
Px is the total byte count, in the priority queue x, can be the strict priority queue of RMAC ports and higher 3 priority
queues for MMAC port. The WRED logic has two drop levels, depending on the value of Px. Each drop level has
defined high-drop and low-drop percentages, which indicate the minimum and maximum percentages of the data
that can be discarded. The X, Y Z percent can be programmed by the register RDRC0, RDRC1. All packets will be
dropped only if the system runs out of the specific buffer resource, per class buffer or per source port buffer. The
WRED thresholds of each queue can be programmed by the QOS control registers (refer to the register group 8).
See Programming QoS Registers application note, ZLAN-42, for more information.
7.4
Shaper
Although traffic shaping is not a primary function of the ZL50404, the chip does implement a shaper for every queue
in the MMAC port. Our goal in shaping is to control the average rate of traffic exiting the ZL50404. If shaper is
enabled, strict priority will be applied to that queue. The priority between two shaped queue is the same as in strict
priority scheduling.
Traffic rate is set using a programmable whole number, no greater than 64. For example, if the setting is 32, then
the traffic rate transmit out of the shaped queue is 32/64 * 1000 Mbps = 500 Mbps. See Programming QoS Register
application note, ZLAN-42, for more information.
36
Zarlink Semiconductor Inc.
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