Re: [PATCH 2/2] mac80211: minstrel_ht: replace rate stats ewma with a better moving average

[Felix Fietkau <nbd@nbd.name>]

On 2019-09-29 20:42, Toke Høiland-Jørgensen wrote:
> Felix Fietkau <nbd@nbd.name> writes:
> 
>> Rate success probability usually fluctuates a lot under normal conditions.
>> With a simple EWMA, noise and fluctuation can be reduced by increasing the
>> window length, but that comes at the cost of introducing lag on sudden
>> changes.
>>
>> This change replaces the EWMA implementation with a moving average that's
>> designed to significantly reduce lag while keeping a bigger window size
>> by being better at filtering out noise.
>>
>> It is only slightly more expensive than the simple EWMA and still avoids
>> divisions in its calculation.
>>
>> The algorithm is adapted from an implementation intended for a completely
>> different field (stock market trading), where the tradeoff of lag vs
>> noise filtering is equally important. It is based on the "smoothing filter"
>> from http://www.stockspotter.com/files/PredictiveIndicators.pdf.
>>
>> I have adapted it to fixed-point math with some constants so that it uses
>> only addition, bit shifts and multiplication
>>
>> To better make use of the filtering and bigger window size, the update
>> interval time is cut in half.
>>
>> For testing, the algorithm can be reverted to the older one via
>> debugfs
> 
> This looks interesting! Do you have any performance numbers from your
> own testing to share? :)
To show the difference, I also generated some random data, ran it
through minstrel's EWMA and the new code and made a plot:
http://nbd.name/ewma-filter-plot.png

The real world test that I did was using mt76x2:
I ran 3 iperf TCP streams from an AP to a station in a cable setup with
an attenuator.
I switched from 70 dB attenuation to 40 dB and measured the time it
takes for TCP throughput to stabilize at a higher rate.
Without my changes it takes about 5-6 seconds, with my changes it's only
2-3 seconds.

- Felix