[Cerowrt-devel] [Bloat] [Codel] FQ_Codel lwn draft article review

[Dan Siemon]

On Tue, 2012-12-04 at 01:23 -0800, Alex Burr wrote:
> [Oops, intended to CC the list.]
> 
> An extra 15-20ms of latency, which is what you're seeing there, 
> shouldn't be caused by packet overhead in the actual VDSL modem part of 
> the device. Worst case the implementation of the PTM-TC layer (the 
> equivalent of ATM) might round the packet up to a multiple of 64 bytes 
> (although it's not supposed to), but unless your line rate is 25kbps, 
> that should not cause 20ms of latency. 
> 
> VDSL2 operates at 4Khz or
>  8Khz symbol rate, and while it contains and interleaving or 
> retransmission layer which can add up to 64ms in its most high-latency 
> configuration, those layers do not operate at the level of packets and 
> I'm pretty sure that that latency cannot be affected by packet size in a
>  conformant implementation.
> This latency is being caused by something else. 
> 
> The
>  best way to figure out the per-packet overhead in your VDSL2 modem is 
> probably  to count the number of packets that get through, not measure 
> latency. 

Thanks for the info. I guess I'll have to keep digging to figure out
where the latency comes from.

I did a couple more experiments which appear to confirm the large amount
of per-packet overhead:
http://www.coverfire.com/archives/2012/12/04/per-packet-overhead-on-vdsl2-2/