[LEDE-DEV] ar71xx: performance decrease with kernel 4.4 (might be due to the qdisc/codel changes)

[Hannu Nyman]

I already said to Felix yesterday that I felt that with kernel 4.4 my ar71xx 
WNDR3800 seemed somehow more sluggish. Now I tested the matter with "flent".

And sadly, with kernel 4.4 my router's performance decreases significantly. 
With kernel 4.1 the router achieves about 20% higher download throughput than 
with the 4.4 build :-(

I used "flent" to measure wired connection throughput with
- two LEDE builds: r241 with kernel 4.1 and r253 with kernel 4.4
- two separate speed limits for SQM simple fq_codel QoS: 85000/10000 kb/s 
that should leave some CPU power free in the router, and 110000/15000 that 
should fully utilise the router's CPU.
- otherwise identical settings, all measurements made inside 30 minutes so no 
changes in traffic conditions

The achieved speeds were:

r241 kernel 4.1 - 85/10: 79 Mb/s down, 8.1 Mb/s up
r253 kernel 4.4 - 85/10: 67 Mb/s down, 8.5 Mb/s up

r241 kernel 4.1 - 110/15: 85 down, 10.5 up
r253 kernel 4.4 - 110/15: 70 down, 10.8 up

(ping latency stays at 16-17 ms with all four tries)

With both SQM speed limits, the kernel 4.1 build performs significantly better.

This performance decrease might be due to the kernel version bump to 4.4 or 
the qdisc/codel changes made to the 4.4 patches a few days earlier.

This chart sums it up:
https://www.dropbox.com/sh/89pntkzjxydnn4c/AAC4x9cScJERL9Wfxm4k43kma?dl=0&preview=Kernel41_44_comparison.png

Full flent data (summary pics & rrul data files) for all four tries is 
available in:
https://www.dropbox.com/sh/89pntkzjxydnn4c/AAC4x9cScJERL9Wfxm4k43kma?dl=0