Reduced throughput with mt7621 and DSA

[Paul D]

I generalize, but this is a problem which affects a few devices in the 
DSA bring-up.

Some platforms require some tinkering and tweaking, and perhaps input 
from the chip-set manufacturer (or information from a datasheet of the 
chip-set) if things are to improve.

DSA brings advantages and is the future: everyone is already committed. 
OEMs have been laying low tho and just used older 2.x kernels which the 
mfrs provided with the chip-sets originally to avoid this maintenance 
themselves if at all possible.

You can have a look at PRs for other platforms and their protracted DSA 
bring-ups on github.

e.g. for the qca8k switch chip, some of the lads have been moving mountains:

https://github.com/openwrt/openwrt/pull/4036
https://github.com/openwrt/openwrt/pull/4622
https://github.com/openwrt/openwrt/pull/4828


This will give you an idea of the amount of work which may be necessary 
to return performance to 19.x swconfig levels.


I don't dabble in mt7621, so cannot be of help there as to whether there 
is something ongoing for those.



On 2021-12-19 12:29, Kristian Evensen wrote:
> Hello,
> 
> I am currently performing some performance measurements, comparing the
> (wired) routing throughput (WAN <-> LAN) of 19.07, 21.02 and master on
> mt7621 (ZBT WG-3526). I have connected one client to my LAN and one to
> the WAN, and use iperf3 to measure. I create parallel flows (in order
> to take advantage of the multiple CPU cores), use TCP and let iperf3
> run for 30 sec. per test.
> 
> Based on my measurements, the throughput is reduced by ~50% going from
> 19.07 and to 21.02/master (~450Mbit/s vs. ~900Mbit/s). I do not have a
> particular commit I can point to, but I believe the regressions is
> caused by the introduction of DSA. Restoring the old swconfig driver,
> brings my 21.02/master throughput up to roughly the same level as
> 19.07.
> 
> I am able to alleviate the reduction in throughput by enabling flow
> offloading, but there are several cases where flow offloading does not
> have an effect. When performing a similar measurement to the one above
> over a Wireguard-tunnel, I see a similar reduction in performance (and
> no help from flow offloading).
> 
> Does anyone know what could be the reason and if there is anything
> that can be done to improve the performance when using DSA? Are there
> for example any out of tree/not yet accepted patches that I should
> try?
> 
> Thanks in advance for any help,
> Kristian
> 
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