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

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Message-ID: <20130530095117.503eeee7@redhat.com>
Date:	Thu, 30 May 2013 09:51:17 +0200
From:	Jesper Dangaard Brouer <jbrouer@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Jesper Dangaard Brouer <brouer@...hat.com>,
	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, 29 May 2013 08:52:04 -0700
Eric Dumazet <eric.dumazet@...il.com> wrote:

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

You are referring to GSO/GRO packets. Yes, one must disable GSO for
this to work. Regardless ATM/ADSL, you should disable GSO when shaping
at low speeds.  Sending 64000 byte on a 512Kbit/s takes approx 1 sec.

http://netoptimizer.blogspot.dk/2010/12/buffer-bloat-calculations.html


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

Are you talking about, that for GSO frames we are not adding a encap
overhead to each "sub" skb.


-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer
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