netdev - Re: tbf/htb qdisc limitations

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Message-ID: <20101012215932.GA1945@del.dom.local>
Date:	Tue, 12 Oct 2010 23:59:32 +0200
From:	Jarek Poplawski <jarkao2@...il.com>
To:	Steven Brudenell <steven.brudenell@...il.com>
Cc:	netdev@...r.kernel.org
Subject: Re: tbf/htb qdisc limitations

On Tue, Oct 12, 2010 at 03:31:48PM -0400, Steven Brudenell wrote:
> > Yes, it's not allowed according to Documentation/HOWTO. Btw, as you
> > can see e.g. in sch_hfsc comments, 64-bit division is avoided too.
> 
> i see sch_hfsc avoids do_div in critical areas for performance
> reasons, but uses it other places. it should still be alright to
> do_div in tbf_change and htb_change_class, right? it would be nice to
> compute the rtabs in those functions instead of having userspace do
> it.

Right, tbf_change or htb_change_class are on the "slow path". But
to compute these rtabs you need passing more parameters than rate.
And userspace would still do most of it for backward compatibility.

> 
> > I can only say there is no versioning, but backward compatibility
> > is crucial, so you need to do some tricks or data duplication.
> > You could probably try to get opinions about it with an RFC on
> > moving tbf and htb schedulers to 64 bits if you're interested
> > (decoupling it from your specific burst problem).
> 
> my burst problem is the only semi-legitimate motivation i can think
> of. the only other possible motivations i can imagine are setting
> "limit" to buffer more than 4GB of packets and setting "rate" to
> something more than 32 gigabit; both of these seem kind of dubious. is
> there something else you had in mind?

No, mainly 10 gigabit rates and additionally 64-bit stats.

> looking more at the netlink tc interface: why is it that the interface
> for so many qdiscs consists of passing a big options struct as a
> single netlink attr, instead of a bunch of individual attrs? this kind
> of seems contrary to the extensibility / flexibility spirit of
> netlink, and seems to be getting in the way of changing the interface.
> maybe i should RFC about this instead ;)

Sure, you can (I'm not the netlink expert).

Jarek P.
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