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

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Message-ID: <5668348.WVIY7FqTii@natalenko.name>
Date:   Sat, 17 Feb 2018 11:01:19 +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 23:59:52 CET Eric Dumazet wrote:
> Well, no effect  here on e1000e (1 Gbit) at least
> 
> # ethtool -K eth3 sg off
> Actual changes:
> scatter-gather: off
> tx-scatter-gather: off
> tcp-segmentation-offload: off
> tx-tcp-segmentation: off [requested on]
> tx-tcp6-segmentation: off [requested on]
> generic-segmentation-offload: off [requested on]
> 
> # tc qd replace dev eth3 root pfifo_fast
> # ./super_netperf 1 -H 7.7.7.84 -- -K cubic
>     941
> # ./super_netperf 1 -H 7.7.7.84 -- -K bbr
>     941
> # tc qd replace dev eth3 root fq
> # ./super_netperf 1 -H 7.7.7.84 -- -K cubic
>     941
> # ./super_netperf 1 -H 7.7.7.84 -- -K bbr
>     941
> # tc qd replace dev eth3 root fq_codel
> # ./super_netperf 1 -H 7.7.7.84 -- -K cubic
>     941
> # ./super_netperf 1 -H 7.7.7.84 -- -K bbr
>     941
> #

That really looks strange to me. I'm able to reproduce the effect caused by 
disabling scatter-gather even on the VM (using iperf3, as usual):

BBR+fq_codel:
sg on:  4.23 Gbits/sec
sg off: 121 Mbits/sec

BBR+fq:
sg on:  6.38 Gbits/sec
sg off: 437 Mbits/sec

Reno+fq_codel:
sg on:  6.74 Gbits/sec
sg off: 1.37 Gbits/sec

Reno+fq:
sg on:  6.53 Gbits/sec
sg off: 1.19 Gbits/sec

Regardless of which congestion algorithm and qdisc is in use, the throughput 
drops, but when BBR is in use, especially with something non-fq, it drops the 
most.

Oleksandr