netdev - Re: TCP and BBR: reproducibly low cwnd and bandwidth

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Date:   Sat, 17 Feb 2018 00:06:23 +0100
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Jerry Chu <hkchu@...gle.com>, Dave Taht <dave.taht@...il.com>
Subject: Re: TCP and BBR: reproducibly low cwnd and bandwidth

On pátek 16. února 2018 23:50:35 CET Eric Dumazet wrote:
> /* snip */
> If you use
> 
> tcptrace -R test_s2c.pcap
> xplot.org d2c_rtt.xpl
> 
> Then you'll see plenty of suspect 40ms rtt samples.

That's odd. Even the way how they look uniformly.

> It looks like receiver misses wakeups for some reason,
> and only the TCP delayed ACK timer is helping.
> 
> So it does not look like a sender side issue to me.

To make things even more complicated, I've disabled sg on the server, leaving 
it enabled on the client:

client to server flow: 935 Mbits/sec
server to client flow: 72.5 Mbits/sec

So still, to me it looks like a sender issue. No?