VSC7440 datasheet(240/444 Pages) MICROSEMI

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Functional Descriptions

VMDS-10492 VSC7440 Datasheet Revision 4.2

223

The following table lists the registers associated with assigning queues for each egress port.

3.24.5.2 Service Statistics

As shown in the following illustration, drop statistics configuration is also being looked up in the mapping

flow. The device has 1,024 service counter sets. On the ingress side, the queue system only counts

discarded frames. On the egress side, it counts the amount of frames transmitted. In both cases, there is

a counter for green frames and another counter for yellow frames.

The egress counter set to count is selected through the rewriter. The ingress counter is selected by

looking up drop statistics information when the queuing class is looked up. That returns a base index,

which is multiplied by four, and one of the four classified priority parameters is added. If the set is

configured for using only four counters, the MSB of the priority is cleared. The resulting index will

afterwards be counting drops, with 18 bits of octet counting, and 10 bits of frame counting. Optionally, all

28 bits can be used for frame counting. For more information about drop statistics, see Statistics,

page 230.

3.24.6

Queue Congestion Control

The congestion control system may choose to forward a frame to a specific queue or to discard it. In

either case, the decision passes through a queue limitation function, which can alter the decision based

on another view of the resources.

The following illustration shows the drop decision flow.

Figure 65 •

Drop Decision Flow

The queue limitation function checks the queue size against various configurable and dynamic values

and may change a transmit request to a discard by canceling the request to the scheduler.

The queue limitation system uses queue sizes (in terms of packet memory usage) of all queues. Each

queue is assigned to a share. The share is a configuration parameter for the port, QoS class, and

service/non-service type as set in the QLIMIT_PORT_CFG register. With this configuration granularity, it

also possible to change the queue rights to be shared for the scheduling element to which they are

attached, so that one aggressive queue will make other queues on the same element start discarding

frames. That is done in the QLIMIT_SHR_MODE field. A set of watermarks is afterwards used to control

some queue limitation drop mechanisms, trying to discard data for congested queues without affecting

well-balanced traffic.

The conditions under which a frame is discarded is shown in the following illustration The watermarks

shown are defined per shared account and per color. If yellow traffic is present, it can be discarded fairly

without affecting the green traffic.

Table 150 •

Queue Mapping Registers Overview

Configuration

Description

QMAP_PORT_MODE

Selects queue layer hierarchy type

QMAP_QOS_TBL

Selects per (qgrp,dport) from which queueing QoS it should be derived

QMAP_SE_TBL

Selects per (qgrp,dport) a scheduling element

Frame

Forwarder

Queue

Mapper

Transmit or

discard frame

copy to a

specific port

Transmit or

discard frame

copy to a

specific queue

Transmit frame

copy to a

specific queue

or skip request

Queue

Limitation

Scheduler

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