LARTC - May 2007 - Problem with the tc statistcis

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

Hi Yang,
>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.
Actually it does:

# tc -s -d qdisc list dev eth1

qdisc pfifo_fast root bands 3 priomap  1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1
 Sent 256404346 bytes 246076 pkt (dropped 0, overlimits 0 requeues 0)
 rate 0bit 0pps backlog 27092b 26p requeues 0
                ^^^^^^^^^^^^^^^^^^
If the counters are set to 0 it is probably because there is no backlog
(i.e., you transmit faster than you queue).
>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?
Are you sure it is not the application that drops the packets?
Did you check the "bytes/pkt" counters on the tx and rx hosts?

Regards
/Christian
(http://benve.info)