netdev - Re: [PATCH net-next 5/5] tcp_cubic: make Hystart aware of pacing

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Date:   Mon, 23 Dec 2019 12:22:03 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH net-next 5/5] tcp_cubic: make Hystart aware of pacing

On Mon, Dec 23, 2019 at 12:20 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> For years we disabled Hystart ACK train detection at Google
> because it was fooled by TCP pacing.
>
> ACK train detection uses a simple heuristic, detecting if
> we receive ACK past half the RTT, to exit slow start before
> hitting the bottleneck and experience massive drops.
>
>
...

> +                       threshold = ca->delay_min;
> +                       /* Hystart ack train triggers if we get ack past
> +                        * ca->delay_min/2.
> +                        * Pacing might have delayed packets up to RTT/2
> +                        * during slow start.
> +                        */
> +                       if ((sk->sk_pacing_status == SK_PACING_NONE) || (netdev_max_backlog & 1))
> +                               threshold >>= 1;

This is silly, last minute debug feature that I forgot to remove

Will send a V2 without it.