[Cake] A few puzzling Cake results

[Toke Høiland-Jørgensen]

Jonathan Morton <chromatix99 at gmail.com> writes:

>>>> your solution significantly hurts performance in the common case
>>> 
>>> I'm sorry - did someone actually describe such a case?  I must have
>>> missed it.
>> 
>> I started this whole thread by pointing out that this behaviour results
>> in the delay of the TCP flows scaling with the number of active flows;
>> and that for 32 active flows (on a 10Mbps link), this results in the
>> latency being three times higher than for FQ-CoDel on the same link.
>
> Okay, so intra-flow latency is impaired for bulk flows sharing a
> relatively low-bandwidth link. That's a metric which few people even
> know how to measure for bulk flows, though it is of course important
> for sparse flows. I was hoping you had a common use-case where
> *sparse* flow latency was impacted, in which case we could actually
> discuss it properly.
>
> But *inter-flow* latency is not impaired, is it? Nor intra-sparse-flow
> latency? Nor packet loss, which people often do measure (or at least
> talk about measuring) - quite the opposite? Nor goodput, which people
> *definitely* measure and notice, and is influenced more strongly by
> packet loss when in ingress mode?

As I said, I'll run more tests and post more data once I have time.

> The measurement you took had a baseline latency in the region of 60ms.

The baseline link latency is 50 ms; which is sorta what you'd expect
from a median non-CDN'en internet connection.

> That's high enough for a couple of packets per flow to be in flight
> independently of the bottleneck queue.

Yes. As is the case for most flows going over the public internet...

> I would take this argument more seriously if a use-case that mattered
> was identified.

Use cases where intra-flow latency matters, off the top of my head:

- Real-time video with congestion response
- Multiple connections multiplexed over a single flow (HTTP/2 or
  QUIC-style)
- Anything that behaves more sanely than TCP at really low bandwidths.

But yeah, you're right, no one uses any of those... /s

> So far, I can't even see a coherent argument for making this tweak
> optional (which is of course possible), let alone removing it
> entirely; we only have a single synthetic benchmark which shows one
> obscure metric move in the "wrong" direction, versus a real use-case
> identified by an actual user in which this configuration genuinely
> helps.

And I've been trying to explain why you are the one optimising for
pathological cases at the expense of the common case.

But I don't think we are going to agree based on a theoretical
discussion. So let's just leave this and I'll return with some data once
I've had a chance to run some actual tests of the different use cases.

-Toke