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Subject: Re: debugging TXQs being empty
To:     Johannes Berg <johannes@sipsolutions.net>,
        =?UTF-8?Q?Toke_H=c3=b8iland-J=c3=b8rgensen?= <toke@redhat.com>
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 <2e01fb1d5b8ff3335220231251f893c01c6c22d3.camel@sipsolutions.net>
From:   Ben Greear <greearb@candelatech.com>
Organization: Candela Technologies
Message-ID: <64a6ae15-b7be-8c27-4e60-4cb3d253f3dc@candelatech.com>
Date:   Thu, 5 Dec 2019 08:57:37 -0800
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On 12/5/19 8:49 AM, Johannes Berg wrote:
> On Thu, 2019-12-05 at 08:37 -0800, Ben Greear wrote:
> 
>>> All this seems to mean that the TCP stack isn't feeding us fast enough,
>>> but is that really possible?
>>
>> Does UDP work better?
> 
> Somewhat, I get about 1020-1030 Mbps. But still a TON of "TXQ of STA ...
> is now empty" messages. Say this run got about 15 per second of those.

It would seem that it is not some issue with TCP stack then?

In general, UDP uses more CPU to send from user-space than TCP
because of TSO, etc.  Sendmmsg can help a bit, but it is a bit painful
to code against so things like iperf do not use it, at least ones I've
looked at.

Can you provide some details on how you are generating this load?

For what it's worth, we've seen about 1.9Gbps download goodput
when using ax200 as a station receiving traffic from 160Mhz AP.
I don't have any reports of > 1Gbps of upload performance though,
not sure our user with the fast AP has done much upload testing...

>> or pktgen?
> 
> I haven't really tried, the setup is a bit complicated ... and it's
> nowhere near me either :)

Yeah, it will likely crash your system unless you apply years-old patches I posted
too :)

But, at least with pktgen, you can be quite sure it is not some slowdown farther up
the stack that is causing the problem.

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com