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Service Queue QoS Policy Commands
cannot be decremented past a depth of 0. This fact drives how the system chooses which BCG is used to
manage the queue bucket update interval.
If a queue's shaping rate is 1Mbps and the threshold (burst limit) is set to 10Kbytes, the maximum amount
of time that can expire before the queue is updated without resulting in a negative bucket depth is 81.92ms.
This can be calculated by taking the number of bits represented by the bucket depth (10Kbytes = 10 * 1,024
* 8 = 81,920 bits) and dividing it by the rate (81,920 bits / 1,000,000 bits per second = 81.92ms). We know
that the queue will not be removed from the scheduler until the PIR bucket depth has equaled or exceeded
the configured burst threshold, so the bucket will be at least 10Kbytes deep. If the system visits the queue
PIR bucket within 81.92ms, the resulting decrement operation will leave the bucket. If the system takes
longer than 81.92ms, the decrement result will be greater than 10Kbytes and part of the decrement result
will be lost. The net result is from less than timely updates is that the queue will not be returned to the
scheduler context fast enough and some shaping bandwidth for the queue will be lost (underrun the shaping
rate).
Each Q2 based forwarding plane maintains 7 Burst Control Groups, each targeting a certain queue bucket
visitation time. A 40ms, 20ms, 10ms, 5ms, 1ms, 500us and 100us BCG is supported. By default, queues are
placed on a BCG based on shaping rate and the queue's burst limit (PIR threshold depth) is set based on the
BCG visitation time and the queue's specified shaping rate. When all shaping queues on a Q2 are left in a
default burst tolerance management state, the system has sufficient BCG visitation resources to ensure that
all queues do not experience inaccurate bucket decrement conditions.
When explicit burst-limit threshold values are defined for a shaping queue, the system picks an appropriate
BCG based on the queue's configured shaping rate and the explicit threshold to find a BCG with the best
target visitation time that results in worst case decrement values that are less than the configured threshold.
However, when a queue is placed on a 'faster' BCG, more visitation resources are consumed and it is
possible that the system will not meet a queue's decrement constraints.
The show qos bcg command allows visibility into a BCG's historic and current visitation time. The system
samples the amount of time it takes each list to visit each of its associated queues once each second and
stores the last 10 samples. It also keeps the longest visitation time seen since the last time the BCG statistics
were cleared, the longest visitation time for the current queue-to-BCG lists associations, calculated longest
visitation time based on maximum scheduling bandwidth and lastly the longest visitation time for an
optionally defined scheduling rate.
With each sample, the system indirectly calculates the amount of scheduling bandwidth based on how much
Q2 resources were diverted from BNG visitation processing. This calculated scheduling bandwidth is useful
since it can be used to evaluate the worst case longest visitation times for each BCG. The calculated
scheduling bandwidth value is stored with the longest seen visitation time and the longest seen visitation
time with the current queue-to-BCG mappings.
Parameters
burst-control-group-name — The burst-control-group-name is required and specifies which globally unique
Burst Control Group will be displayed. If the specified Burst Control Group does not exist, the show
command will fail and the system will return 'The specified BCG does not exist'.
member-queues [at-risk-only] — The member-queues optional keyword is used to include a list of all
queues attached to the specified burst-control-group-name. The optional at-risk-only keyword may be
added to limit the displayed queues to only include queues that are considered 'at-risk' for inaccurate
shaping based on either the 100% worst case scheduling bandwidth for the current queue mappings.
The 100% scheduling bandwidth used in the 'at-risk' determination may be overridden with a specified
scheduling bandwidth by using the exp-util-bw parameter.
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7450 ESS OS Quality of Service Guide
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