[altq 774] Re: altq and hfsc: newbie questions

[Kenjiro Cho <kjc@csl.sony.co.jp>]

Dennis Jun wrote:
> I'm trying out altq with hfsc, but I'm having a difficult time
> understanding it. I've read the documentation several times however it
> doesn't quite make sense to me.
> 
> Specifically, I just want to setup hfsc (or even cbq, but it seems to me
> it can't control borrowing very accurately) with 2 classes. The total
> bandwidth is 45K. Class 1 will have a guaranteed bandwidth of 40K and
> class 2 will have a guaranteed bandwidth of 5K. Furthermore, each class
> can borrow as much bandwidth from each other as they want, as long as it's
> not being used by the owner of the class. Here is a copy of my config:

[snip]

> What is confusing me is pshare. What does pshare mean exactly? Is the the
> amount of bandwidth a class can borrow of the total unused bandwidth? And
> what about grate. Can why can't I make the lan class have a grate of 300K?
> Please bare with me as I am a newbie when it comes to QoS and it's
> concepts. Much thanx in advance.

The concept of the HFSC algorithm is quite different from that of CBQ.
The real-time scheduler in HFSC guarantees not only the bandwidth but
also the delay bounds.

On the other hand, the link-sharing scheduler in HFSC distributes
bandwidth share among classes without bounding delay.
And the link-sharing scheduler alone can provide CBQ's assigned
bandwidth and borrowing.
For example, if class A and class B are children classes of the same
parent class and A has 40% pshare and B has 20% pshare, A gets twice
as much bandwidth as B whether A's current share is within 40% or not.

So, I think you don't need to use grate for you purpose.

For real-time scheduling, You can't assign more than 80% of the link
capacity since 20% is reserved by default.
It is essential to the real-time scheduler that the total guaranteed
bandwidth does not exceed the actual transmission rate in order to
guarantee the delay for each packet.  For example, if a network card
can send 9Mbps for a 10Mbps link, the total real-time traffic should
be less than 9Mbps.

By the way, you can't specify a port range in ALTQ as you did in your
config.

-Kenjiro