[Cerowrt-devel] [Bloat] [Codel] FQ_Codel lwn draft article review
dan at coverfire.com
Tue Dec 4 22:41:14 EST 2012
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.
> 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
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:
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