From d10473d4e3f9d1b81b50a60c8465d6f59a095c46 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Sat, 22 Feb 2014 22:25:57 -0800 Subject: [PATCH] tcp: reduce the bloat caused by tcp_is_cwnd_limited() tcp_is_cwnd_limited() allows GSO/TSO enabled flows to increase their cwnd to allow a full size (64KB) TSO packet to be sent. Non GSO flows only allow an extra room of 3 MSS. For most flows with a BDP below 10 MSS, this results in a bloat of cwnd reaching 90, and an inflate of RTT. Thanks to TSO auto sizing, we can restrict the bloat to the number of MSS contained in a TSO packet (tp->xmit_size_goal_segs), to keep original intent without performance impact. Because we keep cwnd small, it helps to keep TSO packet size to their optimal value. Example for a 10Mbit flow, with low TCP Small queue limits (no more than 2 skb in qdisc/device tx ring) Before patch : lpk51:~# ./ss -i dst lpk52:44862 | grep cwnd cubic wscale:6,6 rto:215 rtt:15.875/2.5 mss:1448 cwnd:96 ssthresh:96 send 70.1Mbps unacked:14 rcv_space:29200 After patch : lpk51:~# ./ss -i dst lpk52:52916 | grep cwnd cubic wscale:6,6 rto:206 rtt:5.206/0.036 mss:1448 cwnd:15 ssthresh:14 send 33.4Mbps unacked:4 rcv_space:29200 Signed-off-by: Eric Dumazet Cc: Yuchung Cheng Cc: Neal Cardwell Cc: Nandita Dukkipati Cc: Van Jacobson Acked-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_cong.c | 3 +-- 1 file changed, 1 insertion(+), 2 deletions(-) diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c index ad37bf18ae4b..2388275adb9b 100644 --- a/net/ipv4/tcp_cong.c +++ b/net/ipv4/tcp_cong.c @@ -290,8 +290,7 @@ bool tcp_is_cwnd_limited(const struct sock *sk, u32 in_flight) left = tp->snd_cwnd - in_flight; if (sk_can_gso(sk) && left * sysctl_tcp_tso_win_divisor < tp->snd_cwnd && - left * tp->mss_cache < sk->sk_gso_max_size && - left < sk->sk_gso_max_segs) + left < tp->xmit_size_goal_segs) return true; return left <= tcp_max_tso_deferred_mss(tp); } -- 2.30.2