[LARTC] Problem with the tc statistcis

[Yang Su <ysu@inf.ethz.ch>]

I had a linux wireless router. I would like to monitor the queue lengh
of the wireless interface. By default, the wifi0 interface is with
pfifo_fast qdisc which does not report backlog packet. I replaced
pfifo_fast with pfifo:
'tc qdisc replace dev wifi0 root pfifo'
Then I use iperf to send UDP pkts faster than the interface can handle
but when I read the qdisc, the result is like:

qdisc pfifo 8007: limit 10p
Sent 46249560 bytes 30600 pkts (dropped 0, overlimits 0)

It is really strange, because over 80% of packets lost is observed by
application layer but by qdisc, there is no packet dropped at all and no
backlog. Any hints for this result?

Thank you,

Yang Su
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