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Date: Thu, 26 Dec 2019 18:23:41 -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 1/5] tcp_cubic: optimize hystart_update() On Mon, Dec 23, 2019 at 3:28 PM Eric Dumazet <edumazet@...gle.com> wrote: > > We do not care which bit in ca->found is set. > > We avoid accessing hystart and hystart_detect unless really needed, > possibly avoiding one cache line miss. > > Signed-off-by: Eric Dumazet <edumazet@...gle.com> > --- Thanks, good idea. > @@ -450,7 +447,7 @@ static void bictcp_acked(struct sock *sk, const struct ack_sample *sample) > ca->delay_min = delay; > > /* hystart triggers when cwnd is larger than some threshold */ > - if (hystart && tcp_in_slow_start(tp) && > + if (!ca->found && hystart && tcp_in_slow_start(tp) && > tp->snd_cwnd >= hystart_low_window) > hystart_update(sk, delay); > } For the many connections that exit the initial slow start due to loss or ECN marks, it seems ca->found would always be 0, so such connections will be at risk for taking a cache miss to fetch 'hystart'. WDYT about further reordering the logic to avoid that risk for such connections, e.g.: - if (hystart && tcp_in_slow_start(tp) && + if (!ca->found && tcp_in_slow_start(tp) && hystart && neal
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