Impact of parameter tx-ring-limit
on average one-way delay and average ipdv
of one EF stream with best-effort traffic
(CISCO 7200)
Goal: study of the impact of parameter tx-ring-limit
on time properties of an EF stream (in particular on one-way delay)
when best-effort traffic is run concurrently.
The tx-ring-limit
is expressed in particles (512 bytes) and the parameter values
deployed in the test are two: the default size (6000 ?) and 5.
Results are compared.
Stream profiles:
- EF: 300 Kbps, variable packet size
- BE: 2.0 Mbps, 200 packs/sec, 1000 bytes per packet
Note: PVC bandwidth is 1 Mbps.
Results in short:
- tx-ring-limit has a great influence on one-way delay. When set to
5, only by that one-way delay decreases greatly.
- tx-ring-limit has a less important impact on ipdv
- the influence of tx-ring-limit on one-way delay is the same for
any buffer-limit size deployed. It's also the same for any packet size,
with the only exception of small packet sizes, in this case in fact
the improvement in one-way delay is less than for packet sizes closer
to the MTU size.
- tx-ring-limit is irrelevant when no background traffic is
generated.
Comments:
- Figure 1 compares one-way delay for different
tx-ring-limit values with and without best-effort traffic.
In the latter case, the two curves coincide, this means that
tx-ring-limit is relevant when multiple different behaviour aggregates
are run in parallel.
In addition, if the parameter is equal to 5, delay greatly
decreases, in particular for large packet sizes of 1024 bytes or more.
The gain in delay is considerable: in the range [60, 120] msec.
However, even in the "good" case when
tx-ring-limit size is equal to 5 part, the observed increase in delay
when best-effort traffic is added, is considerable: approximately
of 20 msec (independently of the packet size).
- Figure 2 and Figure 4
show that tuning tx-ring-limit can help
at optimizing the one-way delay but has a minor effect on ipdv. In fact,
tx-ring-limit helps at minimizing queueing delay since it represents
the size of a (FIFO) queue collecting packets from each of the WFQ queues.
The ipdv curve does not show a regular pattern. The reason why
for packet size = 1024 bytes ipdv is very small, is unknown.
(Note: for any tx-ring-limit values, the average ipdv is high
because of the presence of few high peaks in ipdv.)
- Figure 3 shows that one-way delay still
varies periodically in time for both tx-ring-limit values.
Figure 1: comparison of average one-way delay values
for different tx-ring-limit values with and without best-effort traffic.
Figure 2: comparison of average ipdv values for different tx-ring-limit
values, with and without best-effort traffic.
Figure 3: one-way delay behaviour over time. For both tx-ring-limit values
it varies periodically.
Figure 4: ipdv behaviour over time for different tx-ring-limit values.
Last modified: Dic 01, 1999