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

[Dave Taht]

As a side note, you can do direct comparisons between data sets in
flent, really easily.

Load up all the data files in it, select something less noisy as a
plot type (cdf, totals, or box totals),
then select Data->Add other open data files.

I am *ALWAYS* a fan of looking at the default, detailed graph (all or
all_scaled)first, to see if there was any weirdnesses that would be
missed by the other plot types.

As an example of weirdnesses you can only detect by looking at a
string of test's long term behavior, see the current thread and
pictures on:

https://plus.google.com/u/0/107942175615993706558/posts/WA915Pt4SRN

One of the things I was concerned about going from 4.1 to 4.4 is that
there was a lot of x86_64 based optimization of the routing cache...
and I was certainly concerned about adding more variables to the
fq_codel algorithm, also. I saw pretty bad register usage in cake on
arm when I last looked, I've kind of always wanted people to try -O2
instead of -Os.


On Mon, May 16, 2016 at 1:48 AM, Hannu Nyman <hannu.nyman at iki.fi> 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
>
>
>
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-- 
Dave Täht
Let's go make home routers and wifi faster! With better software!
http://blog.cerowrt.org