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

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Date:   Fri, 16 Feb 2018 18:37:08 +0100
From:   Oleksandr Natalenko <oleksandr@...alenko.name>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Van Jacobson <vanj@...gle.com>, Jerry Chu <hkchu@...gle.com>
Subject: Re: TCP and BBR: reproducibly low cwnd and bandwidth

Hi.

On pátek 16. února 2018 17:25:58 CET Eric Dumazet wrote:
> The way TCP pacing works, it defaults to internal pacing using a hint
> stored in the socket.
> 
> If you change the qdisc while flow is alive, result could be unexpected.

I don't change a qdisc while flow is alive. Either the VM is completely 
restarted, or iperf3 is restarted on both sides.

> (TCP socket remembers that one FQ was supposed to handle the pacing)
> 
> What results do you have if you use standard pfifo_fast ?

Almost the same as with fq_codel (see my previous email with numbers).

> I am asking because TCP pacing relies on High resolution timers, and
> that might be weak on your VM.

Also, I've switched to measuring things on a real HW only (also see previous 
email with numbers).

Thanks.

Regards,
  Oleksandr