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

[Felix Fietkau]

On 2016-05-16 10:48, Hannu Nyman wrote:
> 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
I've also noticed a throughput decrease on other platforms, I don't have
any hard data on that though. I've tried tracking down the source of
this regression, but haven't gotten anywhere with that yet.

Once we're done with more urgent stuff, I plan to do some more work on
optimizing the network stack.

- Felix