Re: [LARTC] HTB ATM MPU OVERHEAD (without any patching)

From: Jason Boxman <jasonb@edseek.com>
To: lartc@vger.kernel.org
Subject: Re: [LARTC] HTB ATM MPU OVERHEAD (without any patching)
Date: Tue, 12 Apr 2005 00:14:54 +0000	[thread overview]
Message-ID: <200504112014.54566.jasonb@edseek.com> (raw)
In-Reply-To: <001001c53ef2$d4a1d2f0$200fa8c0@DELTA>

On Monday 11 April 2005 20:01, Chris Bennett wrote:
<snip>
> The maximum real bandwidth, assuming no waste of data, is 768 * 48 / 53 > 695.  This accounts for the fact that ATM packets are 53 bytes, with bytes
> being overhead.  So that's the overall rate that I'm working with.

Check.

> I then set the MPU to 96 (2 * 48) since the minimum ethernet packet (64
> bytes) uses two ATM packets (each having 48 bytes of data).  I use 48
> instead of 53 here because we already accounted for the ATM overhead in the
> previous calculation.

Makes sense.

> For the overhead I use 24, since nearly each ethernet packet is going to
> end up splitting an ATM packet at some point.  Just going with law of
> averages (instead of a real world statistical analysis), I'm going with
> each ethernet packet wasting (on average) half of an ATM packet.  Again
> using 48 as the ATM packet size (since we accounted for ATM overhead
> already), 48 / 2 = 24.

Indeed.

> A theoretical comparison of sending 10,000 bytes via various packet would
> therefore look like this:
>
<scary numbers>
> Which of course indicates just how much bandwidth small packets waste...

Yeah.

> So, is this logic crap or what?  Should this at least be close to optimum
> or did I forget something important or make an erroneous assumption?

Those are always the thoughts I had.  I never successfully played around with 
overhead or MPU though.  Did you compare results with and without using 
overhead and mpu settings?

<snip>
> One thing I wish I could do would be to query the DSL modem to see exactly
> how much bandwidth usage it is reporting, but unfortunately my ISP now uses
> these crappy new ADSL modems that don't support SNMP :( :(  My old SDSL
> router did, and I miss that feature a lot, but not enough to buy new ADSL
> modems myself.

Yeah, my modem provides no useful information either.

-- 

Jason Boxman
Perl Programmer / *NIX Systems Administrator
Shimberg Center for Affordable Housing | University of Florida
http://edseek.com/ - Linux and FOSS stuff

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