Re: [Cake] A few puzzling Cake results

["Jonas Mårtensson" <martensson.jonas@gmail.com>]

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On Wed, Apr 18, 2018 at 1:25 PM, Toke Høiland-Jørgensen <toke@toke.dk>
wrote:

> Toke Høiland-Jørgensen <toke@toke.dk> writes:
>
> > Jonathan Morton <chromatix99@gmail.com> writes:
> >
> >>> On 17 Apr, 2018, at 12:42 pm, Toke Høiland-Jørgensen <toke@toke.dk>
> wrote:
> >>>
> >>> - The TCP RTT of the 32 flows is *way* higher for Cake. FQ-CoDel
> >>>  controls TCP flow latency to around 65 ms, while for Cake it is all
> >>>  the way up around the 180ms mark. Is the Codel version in Cake too
> >>>  lenient, or what is going on here?
> >>
> >> A recent change was to increase the target dynamically so that at
> >> least 4 MTUs per flow could fit in each queue without AQM activity.
> >> That should improve throughput in high-contention scenarios, but it
> >> does come at the expense of intra-flow latency when it's relevant.
> >
> > Ah, right, that might explain it. In the 128 flow case each flow has
> > less than 100 Kbps available to it, so four MTUs are going to take a
> > while to dequeue...
>
> OK, so I went and looked at the code and found this:
>
>         bool over_target = sojourn > p->target &&
>                            sojourn > p->mtu_time * bulk_flows * 4;
>
>
> Which means that we scale the allowed sojourn time for each flow by the
> time of four packets *times the number of bulk flows*.
>
> So if there is one active bulk flow, we allow each flow to queue four
> packets. But if there are ten active bulk flows, we allow *each* flow to
> queue *40* packets.


I'm confused. Isn't the sojourn time for a packet a result of the total
number of queued packets from all flows?  If each flow were allowed to
queue 40 packets, the sojourn time would be mtu_time * bulk_flows * 40, no?

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