Re: [PATCH 0/2] NET: Accurate packet scheduling for ATM/ADSL

[Krzysztof Matusik <kyf@arterm.pl>]

Dnia środa, 21 czerwca 2006 14:54, Patrick McHardy napisał:
> > I'd love to see this one implemented. I'm using HFSC more than a year and
> > it never provides proper QoS on ATM/ADSL links; low delays can never be
> > achieved even with significant throttling below the h/w link bandwidth.
>
> Mhh .. I trust you picked a proper clocksource and timer frequency?

250Hz or 1000Hz and jiffies (for a SMP machine)
I don't mean I've got big delays though and those are lowest with HFSC, when 
link is satiated.

IMHO since ATM is sending small packets constantly thorough its link (ATM 
specific) encapsulated data defragmentation cannot simply be achieved (or it 
simply isn't done by modems). As a result one can't simply throttle qdisc 
basing on computations like 
UpperLimit = ( bandwidth*(1 - header_bits/frame_size )
having bandwidth specified by ATM link speed rate, as it can be reported by 
modem, and expect it won't ever be congested.
This seems to be main problem. Link asymmetry, window size etc don't affect 
line congestion and so delays that much I suppose.
The same with incorrect timer :-).

> I hacked up a patch yesterday. I want to do a bit more testing before
> posting it, but its hard to really test the effectiveness because even
> without this patch my DSL line already delivers higher throughput and
> much better delay than it should (its a throttled SDSL line sold as
> ADSL). I'll try to do some testing on an ethernet link now.

I'll follow this thread then.

regards