HTB: quantum of class 10001 is big. Consider r2q change问题

  在tc添加class的时候有时会提示：

HTB: quantum of class 10001 is big. Consider r2q change.

根据HTB的官方文档显示，quantum是在可以“借”的情况下，一次可以“借”多少，并且说这个值最好尽量的小，但要大于MTU；而且这个值是不用手动设置，它会根据r2q的值计算出来。

Changing burst will not remove the warning.  r2q is "rate to quantum" is used
to calculate the quantum for each class : quantum = rate / r2q.  Quantum must
be 1500 < quantum < 60000.  Otherwise you will get warnings from the kernel.
Solution : choose r2q so for each class 1500 < quantum < 60000
Or choose the best r2q you can and specify the quantum manually if you add a
class.

> tc qdisc add dev eth0 root handle 1: htb default 10
Default r2q = 10.

> tc class add dev eth0 parent 1: classid 1:1 htb rate 100mbit burst 2k
> tc class add dev eth0 parent 1:1 classid 1:10 htb rate 80mbit ceil 100mbit
> burst 2k
>
> tc class add dev eth0 parent 1:1 classid 1:11 htb rate 128kbit burst 2k
> tc class add dev eth0 parent 1:11 classid 1:21 htb rate 16kbit ceil 56kbit
> burst 2k
> tc class add dev eth0 parent 1:11 classid 1:22 htb rate 16kbit ceil 40kbit
> burst 2k
> tc class add dev eth0 parent 1:11 classid 1:23 htb rate 16kbit ceil 72kbit
> burst 2k
> tc class add dev eth0 parent 1:11 classid 1:24 htb rate 16kbit ceil 64kbit
> burst 2k
> tc class add dev eth0 parent 1:11 classid 1:25 htb rate 16kbit ceil 40kbit
> burst 2k
> tc class add dev eth0 parent 1:11 classid 1:26 htb rate 16kbit ceil 40kbit
> burst 2k
> tc class add dev eth0 parent 1:11 classid 1:27 htb rate 16kbit ceil 32kbit
> burst 2k
> tc class add dev eth0 parent 1:11 classid 1:28 htb rate 16kbit ceil 56kbit
> burst 2k
Smallest rate : 16kbit = 2 kilobyt / r2q (=10) = 200.  And this is < 1500.  So
you get warnings.
Biggest rate : 100mbit = 12.5 mbyte / r2q = 1.2 Mbyte > 60.000.  So you get
warnings. 

If you do
tc qdisc add dev eth0 root handle 1: htb default 10 r2q 1
Smallest rate : 16kbit = 2kilobyte / r2k = 2000.  And this is > 1500.  So no
warnings.
Biggest rate : 100mbit = 12.5 mbyte / r2q = 12.5 Mbyte > 60.000.  So you get
warnings.  But you can overrule the quantum :
tc class add dev eth0 parent 1:1 classid 1:11 htb rate 128kbit burst 2k
quantum 60000

Quantum is used when 2 classes are getting more bandwidth then the rate.   So
it's only important for sharing the remaining bandwidth.  In that case, each
class may send quantum bytes.