[Make-wifi-fast] TCP performance regression in mac80211 triggered by the fq code

Dave Taht dave.taht at gmail.com
Tue Jul 12 08:13:48 EDT 2016

On Tue, Jul 12, 2016 at 12:09 PM, Felix Fietkau <nbd at nbd.name> wrote:
> Hi,
> With Toke's ath9k txq patch I've noticed a pretty nasty performance
> regression when running local iperf on an AP (running the txq stuff) to
> a wireless client.

Your kernel? cpu architecture?

What happens when going through the AP to a server from the wireless client?

Which direction?

> Here's some things that I found:
> - when I use only one TCP stream I get around 90-110 Mbit/s

with how much cpu left over?

> - when running multiple TCP streams, I get only 35-40 Mbit/s total

with how much cpu left over?
context switch difference between the two tests?
tcp_limit_output_bytes is?

got perf?

> - fairness between TCP streams looks completely fine

A codel will get to long term fairness pretty fast. Packet captures
from a fq will show much more regular interleaving of packets,

> - there's no big queue buildup, the code never actually drops any packets

A "trick" I have been using to observe codel behavior has been to
enable ecn on server and client, then checking in wireshark for ect(3)
marked packets.

> - if I put a hack in the fq code to force the hash to a constant value

You could also set "flows" to 1 to keep the hash being generated, but
not actually use it.

> (effectively disabling fq without disabling codel), the problem
> disappears and even multiple streams get proper performance.

Meaning you get 90-110Mbits ?

Do you have a "before toke" figure for this platform?

> Please let me know if you have any ideas.

I am in berlin, packing hardware...

> - Felix
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Dave Täht
Let's go make home routers and wifi faster! With better software!

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