Re: [LARTC] Some questions remaining about TC

From: Stef Coene <stef.coene@docum.org>
To: lartc@vger.kernel.org
Subject: Re: [LARTC] Some questions remaining about TC
Date: Wed, 11 Jun 2003 16:47:53 +0000	[thread overview]
Message-ID: <marc-lartc-105535012524893@msgid-missing> (raw)
In-Reply-To: <marc-lartc-105519389407466@msgid-missing>

On Wednesday 11 June 2003 12:26, Emmanuel SIMON wrote:
> First, thank you Stef for answering.
> That's what I understood. I think my question was not enough precise (??)
>
> My reel question/problem is why are parameters of the bandwidth (such as
> rate, burts ...) set on classes rather than on qdiscs ?
You have to see a class as a virtual channel.  Each class is responsible for 
the packets that it holds.  So the rate / burst / ceil and so one are only 
valid for that class.  So you have to put packets in the same class that 
somehow are the same.  Like all telnet/ssh packets.  So you can give that 
class a small burst and a low latency.  Or you put the packets from 1 client 
in 1 class so all the traffic to the client is one 1 big channel and he is 
reponsible for this.

> I guess a class should only map a flow and should not know how this flow
> will be sent. Especially that classes don't send packets. What is the
> reason why it is done so ? And what if i don't specify parameters fir a
> leaf class ? Does it herit from the parent ?
A class is not reponsible for sending the packets because the class is only 
used to create the virtual channels (and so to limit the packets in that 
class).  Once a packet is dequeued from a virtual channel (and so is dequeued 
form a class) it's give to the qdisc attached to that class.  So you can add 
a small fifo qdisc to that class so packets are send fast.  Or you can add a 
sfq qdisc so each flow has his own little queue.  So you have a lot of 
flexibility.

You can even add a classfull qdisc to a class, but that's just a waste of CPU 
cycles and will only add extra delays.

Each class needs it's own parameters when you create it.  It will not inheret 
something from its parent.

> Thanks once more
This is even for more confusing when I have to think about ut.  This is in no 
docs explained, but I don't think that I'm telling big lies.

Stef

-- 

stef.coene@docum.org
 "Using Linux as bandwidth manager"
     http://www.docum.org/
     #lartc @ irc.oftc.net

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