Re: [LARTC] Re: questiona about CBQ algorithm in Linux

[Martin Devera <devik@cdi.cz>]

Hi yuxiao,

On Mon, 9 Sep 2002, yuxiao wrote:

> Hello Alex
>
> Sorry for my stubborn question. I read the S.Floyd paer "Link-sharing and
> resource mamagement...." On page 5, they said" The estimator estimates the
> bandwidth used byeach class over the appropriate time interval, to determine
> whether or not each class has been receiveing its link -sharing
> bandwidth..."
>
> Also as I know CBQ building block, they have general scheduler, link-sharing
> scheduler, estimater, classifier etc. complonents.
>
> This is why I ask you how about its implementation in linux. I read the
> sch_cbq.c comments, not quite sure one line" Linux has no EOI event, so that
> we ca not estimater true idle class idle time..."
> what's EOI event, This is why in Linux, CBQ canot measure bandwidth?

Alexey meant EndOfInterrupt probably - the timepoint when
a packet finally leaved NIC. It is needed to compute and
measure inderpacket times - these are used as guide whether
are we over/underlimit.
When I started to write first HTB version I spent one YEAR
of thinking - so that don't expect to understand all nuances
so fast :)
The main problems when implementing CBQ as described in Floyd
paper (Appendix A - interpacked delay measure) is that you
don;t have precise time reference. Alexey used trick in sch_cbq
- used NIC as time source ;)
HTB is direct implementation of Floyd's formal sharing and
uses leaky buckets as estimators - they are not so sensitive
to less precise timing.
Also you should note that original CBQ really didn't measure
bw, it rather tries to correct deviations from expected interpacket
delay.

regards, devik

> regards
>
> yuxiao
> ----- Original Message -----
> From: <kuznet@ms2.inr.ac.ru>
> To: "yuxiao" <yxjia@site.uottawa.ca>
> Cc: <lartc@mailman.ds9a.nl>; <stef.coene@docum.org>
> Sent: Friday, September 06, 2002 11:31 AM
> Subject: Re: questiona about CBQ algorithm in Linux
>
>
> > Hello!
> >
> > > average), how about  Linux implemenatation about estimator? 
> > > also do you have link for this algorithm? I canot find it.
> >
> > The questions are answered in papers, references are in comments
> > in sch_cbq.c, look at http://www.icir.org/floyd/cbq.html
> >
> >
> > > 3. In linux how CBQ measure used bandwidth per class? does it measure
> > > bandwith bits/sec?
> >
> > No, it does not measure bandwdith at all. Read papers, please.
> >
> >
> > > 7. Can HTB do the same job as CBQ right now, can HTB do borrow ? 
> >
> > Yes, it does. And does it better than CBQ.
> >
> > Alexey
>
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