HTB accuracy for high speed

[Antonio Almeida <vexwek@gmail.com>]

Hi!
I've been using HTB in a Linux bridge and recently I noticed that, for
high speed, the configured rate/ceil is not respected as for lower
speeds.
I'm using a packet generator/analyser to inject over 950Mpbs, and see
what returns back to it, in the other side of my bridge. Generated
packets have 800bytes. I noticed that, for several tc HTB rate/ceil
configurations the amount of traffic received by the analyser stays
the same. See this values:

HTB conf      Analyser reception
476000Kbit    544.260.329
500000Kbit    545.880.017
510000Kbit    544.489.469
512000Kbit    546.890.972
-------------------------
513000Kbit    596.061.383
520000Kbit    596.791.866
550000Kbit    596.543.271
554000Kbit    596.193.545
-------------------------
555000Kbit    654.773.221
570000Kbit    654.996.381
590000Kbit    655.363.253
605000Kbit    654.112.017
-------------------------
606000Kbit    728.262.237
665000Kbit    727.014.365
-------------------------

There are these steps and it looks like doesn't matter if I configure
HTB to 555Mbit or to 605Mbit - the result is the same: 654Mbit. This
is 18% more traffic than the configured value. I also realise that for
smaller packets it gets worse, reaching 30% more traffic than what I
configured. For packets of 1514bytes the accuracy is quiet good.
I'm using kernel 2.6.25

My 'tc -s -d class ls dev eth1' output:

class htb 1:10 parent 1:2 rate 1000Mbit ceil 1000Mbit burst 126375b/8
mpu 0b overhead 0b cburst 126375b/8 mpu 0b overhead 0b level 5
 Sent 51888579644 bytes 62067679 pkt (dropped 0, overlimits 0 requeues 0)
 rate 653124Kbit 97656pps backlog 0b 0p requeues 0
 lended: 0 borrowed: 0 giants: 0
 tokens: 113 ctokens: 113

class htb 1:1 root rate 1000Mbit ceil 1000Mbit burst 126375b/8 mpu 0b
overhead 0b cburst 126375b/8 mpu 0b overhead 0b level 7
 Sent 51888579644 bytes 62067679 pkt (dropped 0, overlimits 0 requeues 0)
 rate 653123Kbit 97656pps backlog 0b 0p requeues 0
 lended: 0 borrowed: 0 giants: 0
 tokens: 113 ctokens: 113

class htb 1:2 parent 1:1 rate 1000Mbit ceil 1000Mbit burst 126375b/8
mpu 0b overhead 0b cburst 126375b/8 mpu 0b overhead 0b level 6
 Sent 51888579644 bytes 62067679 pkt (dropped 0, overlimits 0 requeues 0)
 rate 653124Kbit 97656pps backlog 0b 0p requeues 0
 lended: 0 borrowed: 0 giants: 0
 tokens: 113 ctokens: 113

class htb 1:108 parent 1:10 leaf 108: prio 7 quantum 1514 rate
555000Kbit ceil 555000Kbit burst 70901b/8 mpu 0b overhead 0b cburst
70901b/8 mpu 0b overhead 0b level 0
 Sent 51888579644 bytes 62067679 pkt (dropped 27801917, overlimits 0 requeues 0)
 rate 653124Kbit 97656pps backlog 0b 0p requeues 0
 lended: 62067679 borrowed: 0 giants: 0
 tokens: -798 ctokens: -798

As you can see, class htb 1:108 rate's is 653124Kbit! Much bigger that
it's ceil.

I also note that, for HTB rate configurations over 500Mbit/s on leaf
class, when I stop the traffic, in the output of "tc -s -d class ls
dev eth1" command, I see that leaf's rate (in bits/s) is growing
instead of decreasing (as expected since I've stopped the traffic).
Rate in pps is ok and decreases until 0pps. Rate in bits/s increases
above 1000Mbit and stays there for a few minutes. After two or three
minutes it becomes 0bit. The same happens for it's ancestors (also for
root class).Here's tc output of my leaf class for this situation:

class htb 1:108 parent 1:10 leaf 108: prio 7 quantum 1514 rate
555000Kbit ceil 555000Kbit burst 70901b/8 mpu 0b overhead 0b cburst
70901b/8 mpu 0b overhead 0b level 0
 Sent 120267768144 bytes 242475339 pkt (dropped 62272599, overlimits 0
requeues 0)
 rate 1074Mbit 0pps backlog 0b 0p requeues 0
 lended: 242475339 borrowed: 0 giants: 0
 tokens: 8 ctokens: 8


  Antonio Almeida