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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
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