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TC: Dropped vs Overlimit packets



Title: TC: Dropped vs Overlimit packets

Hi,

What is the difference between dropped and overlimit packets?  I have a CBQ qdisc, with two classes, two filters and a SFQ qdisc attached to each class.  The root qdisc and each class have 10Mbit as bandwidth, and the classes have a rate of 30kbit.  When I look at stats, I get the following:

root@rtr13:/tools/bin> tc -s qdisc;tc -s class show dev eth3
qdisc sfq 800f: dev eth3 quantum 1514b perturb 15sec
 Sent 10064064 bytes 157251 pkts (dropped 9450737, overlimits 0)

 qdisc sfq 800e: dev eth3 quantum 1514b perturb 15sec
 Sent 10064128 bytes 157252 pkts (dropped 3960483, overlimits 0)
 backlog 1p

 qdisc cbq 1: dev eth3 rate 10Mbit (bounded,isolated) prio no-transmit
 Sent 20169814 bytes 315164 pkts (dropped 13411220, overlimits 13863980)
 backlog 1p
  borrowed 0 overactions 0 avgidle 624 undertime 0

 class cbq 1: root rate 10Mbit (bounded,isolated) prio no-transmit
 Sent 20169750 bytes 315163 pkts (dropped 0, overlimits 0)
  borrowed 0 overactions 0 avgidle 624 undertime 0
class cbq 1:1 parent 1: leaf 800e: rate 50Kbit (bounded) prio 1
 Sent 10064064 bytes 157251 pkts (dropped 3960483, overlimits 13930387)
 backlog 1p
  borrowed 0 overactions 153157 avgidle -108 undertime -78
class cbq 1:2 parent 1: leaf 800f: rate 50Kbit (bounded) prio 1
 Sent 10064064 bytes 157251 pkts (dropped 9450737, overlimits 14057602)
  borrowed 0 overactions 153157 avgidle -26 undertime 8179

I can see the dropped packets for the two SFQ qdiscs total the dropped packets for the root qdisc, but what is the meaning of the overlimit number?  If the packets were over the 30kbit rate within each class, wouldn't they be added to the drop count?  How does the class determine what number to count (sent, dropped or overlimit) for each packet?

Jon