[Bloat] How about a topical LWN article on demonstrating the real-world goodness of CAKE?

[Sebastian Moeller]

Hi Tom,


On 10 August 2020 17:08:49 CEST, Tom Henderson <tomh at tomh.org> wrote:
>
>> so after much tweaking, I've got cake set to 40mbit down, 20mbit up,
>> enforced by two cakes (one for each NIC). that's fairly low --
>
>Thanks for sharing this configuration experience, but it leads me back 
>to a question I have about best current practice for deployment.  Can 
>CAKE/SQM handle dynamic Wi-Fi bandwidth due to Wi-Fi rate control 
>selecting lower MCS to increase range, or does it rely on first getting
>
>the Wi-Fi deployed so that it has strong signal everywhere, and then 
>finding a CAKE shaping rate that shaves off a few Mb/s from the highest
>
>capacity MCS so that the bottleneck always lands on the CAKE AQM?  It 
>seems like the deployment that you shared with a separate router will 
>require a predictable Wi-Fi rate, but I am wondering more about the
>case 
>in which CAKE is deployed on the AP.
>
>- Tom

No, neither cake nor SQM can deal all too well with variable rate links, if we talk about link rate variations in the second to second range. But in OpenWrt both the ath9k and the ath10k WiFi drivers gained airtime fairness modes, which do a pretty good job at keeping link sharing fair and WiFi bufferbloat low. 
The issue here is not really that closely related to cake or sqm, but that WiFi without AQL (airtime queueing limits, analog to wired Ethernet's byte queue limits) does not give the required pushback for an upstream qdisc to keep the WiFi queues at an acceptable/reasonable size and it also does not really offer a reliable fast estimator of achievable rate, as far as I can tell.
The current best practice seems to be to instantiate cake/SQM on a reasonably fixed rate wan link and select WiFi cards/socs that offer decent airtime fairness.
Works pretty well in practice...

Best regards
        Sebastian


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