Re: [PATCH 2/2] NET: Accurate packet scheduling for ATM/ADSL (userspace)

[Jesper Dangaard Brouer <hawk@comx.dk>]

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On Wed, 2006-06-14 at 11:57 +0100, Alan Cox wrote:
> Ar Mer, 2006-06-14 am 11:40 +0200, ysgrifennodd Jesper Dangaard Brouer:
> > option to calculate traffic transmission times (rate table)
> > over all ATM links, including ADSL, with perfect accuracy.
>
> The other problem I see with this code is it is very tightly tied to ATM
> cell sizes, not to solving the generic question of packetisation. 

Well, we did consider to do so, but we though that it would be harder to
get it into the kernel.

Actually thats the reason for the defines:
 #define        ATM_CELL_SIZE           53
 #define        ATM_CELL_PAYLOAD        48

Changing these should should make it possible to adapt to any other SAR
(Segment And Reasembly) link layer.

> I'm
> not sure if that matters but for modern processors I'm also sceptical
> that the clever computation is actually any faster than just doing the
> maths, especially if something cache intensive is also running.

I guess you are refering to the rate table lookup system, that is based
upon array lookups.  I do think that the rate table array lookup system
has been outdated, as memory access is the bottleneck on modern CPUs.
But its design by Alexey for a long time ago where the hardware
restrictions were different.  It also avoids floting point operations in
the kernel.

Thanks for your comments.

-- 
Med venlig hilsen / Best regards
  Jesper Brouer
  ComX Networks A/S
  Linux Network developer
  Cand. Scient Datalog / MSc.
  Author of http://adsl-optimizer.dk


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