netdev - Re: tc linklayer ADSL calc broken after commit 56b765b79 (htb: improved accuracy at high rates)

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Message-ID: <1369842724.5109.44.camel@edumazet-glaptop>
Date:	Wed, 29 May 2013 08:52:04 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jesper Dangaard Brouer <brouer@...hat.com>
Cc:	Stephen Hemminger <stephen@...workplumber.org>,
	David Miller <davem@...hat.com>, j.vimal@...il.com,
	Michal Soltys <soltys@....info>,
	Mike Frysinger <vapier@...too.org>,
	Jussi Kivilinna <jussi.kivilinna@...et.fi>,
	Patrick McHardy <kaber@...sh.net>,
	Jiri Pirko <jpirko@...hat.com>,
	Toke Høiland-Jørgensen <toke@...e.dk>,
	Dave Taht <dave.taht@...il.com>, netdev@...r.kernel.org,
	bloat@...ts.bufferbloat.net, Dan Siemon <dan@...erfire.com>,
	Jim Gettys <jg@...edesktop.org>,
	Steven Barth <cyrus@...nwrt.org>, Felix Fietkau <nbd@....name>,
	Jiri Benc <jbenc@...hat.com>
Subject: Re: tc linklayer ADSL calc broken after commit 56b765b79 (htb:
 improved accuracy at high rates)

On Wed, 2013-05-29 at 15:13 +0200, Jesper Dangaard Brouer wrote:
> I recently discovered that the (traffic control) tc linklayer
> calculations for ATM/ADSL have been broken by:
>  commit 56b765b79 (htb: improved accuracy at high rates).
> 
> Thus, people shaping on ADSL links, using e.g.:
>  tc class add ... htb rate X ceil Y linklayer atm overhead 10
> 
> Will no-longer get ATM cell tax/overhead adjusted.
> 
> How can we solve/fix this?
> Perhaps we can change to use the "stab" system instead (as it does
> not seem to be broken by the commit).
> 
> But how do we facilitate a change to use "stab" system (for all the
> scripts using the old option)?
> 
> Can we change the iproute2/tc command to handle this transparently, or
> should we give an error/warning if someone uses "tc" and "linklayer" on
> a kernel above v.3.8. ?
> 
> 
> History:
>  - My linklayer ATM changes appeared in kernel 2.6.24 (and iproute2 2.6.25)
>  - The STAB changes appeared in kernel 2.6.27
> 

Hi Jesper

stab suffers from the same problem : its table driven, so works only for
packet smaller than a given size.

I am not sure it will solve the ATM logic (with the 5 bytes overhead per
48 bytes cell)

btw, even on old kernels :


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