Re: [LARTC] how to get the latency down on maxed out classes?

[Stef Coene <stef.coene@docum.org>]

On Saturday 07 December 2002 14:15, Abraham van der Merwe wrote:
> Hi!
>
> I'm using HTB to shape traffic to/from clients, but one of the problems I
> have is that once a class utilizes its maximum potential, they latency
> spirals out of control.
>
> For example:
>
>      .-----.
>
>      | isp |
>
>      `-----'
>
>    .--------.
>
>    | shaper |
>
>    `--------'
>
>    .--------.
>
>    | client |
>
>    `--------'
>
> lets say I want to limit traffic to/from client to 64kbit. now, client
> opens a tcp connection blasting away at full speed.
>
> If client now pings isp, it gets on average around 7 seconds latency. I
> tried to improve this by using SFQ on the leaf nodes of my HTB hierarchy,
> but that does not really improve the situation, only makes it much worse.
> with SFQ I get anything between 250ms and 13 seconds latency.
>
> I then tried fifos. With small packet fifos the packet loss is just
> to great to be of any use and even then the latency is quite high (~200ms).
> If I make the fifos big enough so that unreasonable numbers of packets
> isn't dropped, it just doesn't do much to the latency/or throughput. This
> behaviour kind of makes sense, but doesn't help me :P
>
> I'm thinking of using RED, but the number of parameters is daunting and I
> have no idea how the HTB rate correlates to packet size and burst rates for
> red.
>
> Does anybody have any idea how to get the latency down and still maintain
> the correct throughput?
If you want low latency for some traffic (ping, telnet, ssh), then you can 
create a separate class for it. 
And if you have a 64kbit modem, you have to be sure you never send more data 
then the modem can handle.  If you send more data, the hugh modem queue's 
will be filled so they create a lot of latency.  So for a 64kbit link, try to 
limit ALL traffic to 60kbit so the queue's of the modem are never filled.

Stef

-- 

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

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