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

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Date:   Fri, 16 Feb 2018 23:50:55 +0100
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.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

Hi.

On pátek 16. února 2018 21:54:05 CET Eric Dumazet wrote:
> /* snip */
> Something fishy really :
> /* snip */
> Not only the receiver suddenly adds a 25 ms delay, but also note that
> it acknowledges all prior segments (ack 112949), but with a wrong ecr
> value ( 2327043753 )
> instead of 2327043759
> /* snip */

Eric has encouraged me to look closer at what's there in the ethtool, and I've 
just had a free time to play with it. I've found out that enabling scatter-
gather (ethtool -K enp3s0 sg on, it is disabled by default on both hosts) 
brings the throughput back to normal even with BBR+fq_codel.

Whyyyy? What's the deal BBR has with sg?

Oleksandr