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Date:   Thu, 12 Dec 2019 16:47:30 +0100
From:   Johannes Berg <>
To:     Neal Cardwell <>
Cc:     Eric Dumazet <>,
        Toke Høiland-Jørgensen <>,, Netdev <>
Subject: Re: debugging TCP stalls on high-speed wifi

Hi Neal,

On Thu, 2019-12-12 at 10:11 -0500, Neal Cardwell wrote:
> On Thu, Dec 12, 2019 at 9:50 AM Johannes Berg <> wrote:
> > If you have any thoughts on this, I'd appreciate it.
> Thanks for the detailed report!

Well, it wasn't. For example, I neglected to mention that I have to
actually use at least 2 TCP streams (but have tried up to 20) in order
to not run into the gbit link limit on the AP :-)

> I was curious:
> o What's the sender's qdisc configuration?

There's none, mac80211 bypasses qdiscs in favour of its internal TXQs
with FQ/codel.

> o Would you be able to log periodic dumps (e.g. every 50ms or 100ms)
> of the test connection using a recent "ss" binary and "ss -tinm", to
> hopefully get a sense of buffer parameters, and whether the flow in
> these cases is being cwnd-limited, pacing-limited,
> send-buffer-limited, or receive-window-limited?

Sure, though I'm not sure my ss is recent enough for what you had in
mind - if not I'll have to rebuild it (it was iproute2-ss190708).

Note there are 4 connections (iperf is being used) but two are control
and two are data. Easy to see the difference really :-)

> o Would you be able to share a headers-only tcpdump pcap trace?

I'm not sure how to do headers-only, but I guess -s100 will work.

Note that this is from a different run than the ss stats.


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