Re: [LARTC] Determining tbf parameters

From: Kurt Wagner <kwagner@cosy.sbg.ac.at>
To: lartc@vger.kernel.org
Subject: Re: [LARTC] Determining tbf parameters
Date: Thu, 28 Mar 2002 17:09:42 +0000	[thread overview]
Message-ID: <marc-lartc-101733528311936@msgid-missing> (raw)
In-Reply-To: <marc-lartc-101682297718443@msgid-missing>

Howdi Alex,

On Fri, 22 Mar 2002, Alex Leyva wrote:

> I've been triying with diferent values for burst, limit and latency, but
> we cant understand how to determine the correct values for them.
> 
> In "Short evaluation of linux token-bucket-filter queueing discipline"
> by Kurt Wagner we learned that queue size=limit+bucket size, but we dont
> know how to determine limit, burst and latency.

I state that the queue size is equal to limit MINUS the bucket size, not 
PLUS the bucket size. Whereas queue size is no configurable parameter in 
TBF, I thought it would better describe TBF's shaping/dropping behavior if 
I write it like that.

Regarding the way to set burst, limit and latency I wan't to remind that 
you don't have to find values for all of them, as latency is approximately 
(limit - burst) / rate.

So TBF give's you the freedom of choice. You can choose burst and limit, 
or burst and latency to configure it, as the HOWTO states in chapter 9.2.

Now, how to set it depends on what you want to do. Which bursts do you 
wan't to allow, and do you wan't to queue packets when the bucket is empty 
(introducing additional delay...) or simply drop them?

> We found different formulas to determine the size of the bucket, the
> latency and the limit, but they doesnt work for us, because we get
> incoherent values.

If you have some rules of thumb for some sort of setup please post it, 
could be interesting for other people too. However, the general rule is 
that there is no general rule. ;-)

> We have another question, what is the normal values for dropped and
> overlimit?.
> We have:
> qdisc tbf 110: dev eth1 rate 1Mbit burst 20Kb/8 mpu 0b [0001f400] limit
> 40Kb lat 190.7ms
>  Sent 17542230 bytes 11591 pkts (dropped 36, overlimits 35216)
> 
> is this normal?.

Hmm, 17MByte transmitted with 11591 packets, so your packets are 
approximately 1500 bytes long in average, which makes me guess it 
was a file transfer with TCP and 1.5KB MTU? The values are ok if 
that's the case...

bye,
Kurt

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