netdev - Re: [PATCH net-next v2 3/5] tcp_cubic: switch bictcp

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Date:   Fri, 27 Dec 2019 11:08:31 -0500
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH net-next v2 3/5] tcp_cubic: switch bictcp_clock() to usec resolution

On Mon, Dec 23, 2019 at 3:28 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> Current 1ms clock feeds ca->round_start, ca->delay_min,
> ca->last_ack.
>
> This is quite problematic for data-center flows, where delay_min
> is way below 1 ms.
>
> This means Hystart Train detection triggers every time jiffies value
> is updated, since "((s32)(now - ca->round_start) > ca->delay_min >> 4)"
> expression becomes true.
>
> This kind of random behavior can be solved by reusing the existing
> usec timestamp that TCP keeps in tp->tcp_mstamp
>
> Note that a followup patch will tweak things a bit, because
> during slow start, GRO aggregation on receivers naturally
> increases the RTT as TSO packets gradually come to ~64KB size.
>
> To recap, right after this patch CUBIC Hystart train detection
> is more aggressive, since short RTT flows might exit slow start at
> cwnd = 20, instead of being possibly unbounded.
>
> Following patch will address this problem.
>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---

Acked-by: Neal Cardwell <ncardwell@...gle.com>

Thanks!
neal