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

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2013-05-29 at 15:50 -0700, Stephen Hemminger wrote:
> On Wed, 29 May 2013 08:52:04 -0700
> Eric Dumazet <eric.dumazet@gmail.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).
> > > 
> > > 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 :
> 
> 
> How bad is the failure? If it is fixed, will it break existing installations?
> 
> Which probably means, is anyone but the original developers ever using it
> and therefore likely to notice?

Adding the logic on the kernel is doable, by adding some clean
attributes so that tc can setup the feature, and report the attributes
back.

cpus are fast today and can perform the atm cell/overhead faster than a
table lookup.