On Thu, 2006-03-02 at 14:51 +0100, Markus Schulz wrote:
> > Why you don't use the existing overhead parameter? It's useless to
> > have two parameters which do the exact same thing (existing overhead
> > and your atm).
> > Only ATM Cell alignment must be added to rate table calculation.
The overhead and atm options don't do the "exact same
thing". If the atm option is present, tc includes the
atm cell alignment overheads in the rate table
calculation. Otherwise it doesn't.
As atm cell overheads aren't a fixed amount (they vary
in a non-linear fashion between 6 and 202 bytes), you
can't use the overhead option to calculate them.
> But it would be nice if this would be patched into upstream iproute
> source. Then there is no need of patching for qos at adsl links.
Yes.
After posting my patch I went away and had a think,
and realised that because of rounding issues my
calculation of atm cell overhead would be wrong in
some cases. The only fix I could think of was to
modify the kernel.
I was not aware of Jesper's adsl-optimiser patches
until you pointed them out just now. Having looked
at them two things stand out:
a. Jesper had already come across the rate calculation
problem and had solved it. His solution and my
intended solution are the same.
The problem is caused by the scaling of the packet
size prior to looking up the rate table. The
easy solution is to add the overhead in the kernel,
rather than having tc include the overhead in the
pre-calculated rate table. This is what Jesper's
kernel patch does.
(The rate table is indexed by packet size, and
returns the amount of time required to send a packet
of that size. It is a fixed size table of 256
entries (0..255). Since packets can be bigger than
255 bytes long, the packet size is first scaled so
it will fit into the 0..255 range. Scaling is
achieved by dividing the packet size by a power of 2
(ie 1, 2, 4, 8, etc). A scale factor of 8 handles
packet sizes of 1024..2047, so this is what most
links end up using.)
If you don't patch the kernel, the rate calculated by
the kernel will be wrong around 10% of the time (as
opposed to very nearly 100% of the time if you don't
use the patch at all).
2. Jasper didn't make including the atm cell overhead
optional - so it is included for all links, whether
they use atm or not. Thus he doesn't have an "atm"
parameter. This means it isn't general purpose, and
could not be distributes as part of the standard tc
package.
In any case, apart from those two relatively minor
differences the two patches are the identical in what
they achieve and how they do it.
Personally, I didn't care much about any of this
before VOIP because when it is all said and done, the
3% error you get for large packet sizes is easily
accounted for by just adjusting the rate accordingly.
That doesn't work when the error rate rises to 40%, as
it does for VOIP. As VOIP is going to become a
significant part of internet traffic in the not too
distant future, I think it is time to consider including
some combination of these patches into the main line.
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