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> On 18 Apr 2018, at 12:25, Toke Høiland-Jørgensen <[email protected]> wrote:
>
> Toke Høiland-Jørgensen <[email protected]> writes:
>
>> Jonathan Morton <[email protected]> writes:
>>
>>>> On 17 Apr, 2018, at 12:42 pm, Toke Høiland-Jørgensen <[email protected]> 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.
>
> This completely breaks the isolation of different flows, and makes the
> scaling of Cake *worse* than plain CoDel.
>
> So why on earth would we do that?
The thread that lead to that change:
https://lists.bufferbloat.net/pipermail/cake/2017-December/003159.html
Commits: 0d8f30faa3d4bb2bc87a382f18d8e0f3e4e56eac & the change to 4*bulk flows
49776da5b93f03c8548e26f2d7982d553d1d226c
Cheers,
Kevin D-B
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