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 <edumazet@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Cc: Van Jacobson <vanj@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index ad37bf1..2388275 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -290,8 +290,7 @@
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);
}