Commit da882c1f authored Apr 22, 2012 by Eric Dumazet Committed by David S. Miller Apr 23, 2012

tcp: sk_add_backlog() is too agressive for TCP



While investigating TCP performance problems on 10Gb+ links, we found a
tcp sender was dropping lot of incoming ACKS because of sk_rcvbuf limit
in sk_add_backlog(), especially if receiver doesnt use GRO/LRO and sends
one ACK every two MSS segments.

A sender usually tweaks sk_sndbuf, but sk_rcvbuf stays at its default
value (87380), allowing a too small backlog.

A TCP ACK, even being small, can consume nearly same truesize space than
outgoing packets. Using sk_rcvbuf + sk_sndbuf as a limit makes sense and
is fast to compute.

Performance results on netperf, single flow, receiver with disabled
GRO/LRO : 7500 Mbits instead of 6050 Mbits, no more TCPBacklogDrop
increments at sender.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Maciej Żenczykowski <maze@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Cc: Rick Jones <rick.jones2@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent f545a38f

net/ipv4/tcp_ipv4.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -1752,7 +1752,8 @@ process:
		if (!tcp_prequeue(sk, skb))
		ret = tcp_v4_do_rcv(sk, skb);
		}
		} else if (unlikely(sk_add_backlog(sk, skb, sk->sk_rcvbuf))) {
		} else if (unlikely(sk_add_backlog(sk, skb,
		sk->sk_rcvbuf + sk->sk_sndbuf))) {
		bh_unlock_sock(sk);
		NET_INC_STATS_BH(net, LINUX_MIB_TCPBACKLOGDROP);
		goto discard_and_relse;

net/ipv6/tcp_ipv6.c

+2 −1

Original line number	Diff line number	Diff line
		@@ -1654,7 +1654,8 @@ process:
		if (!tcp_prequeue(sk, skb))
		ret = tcp_v6_do_rcv(sk, skb);
		}
		} else if (unlikely(sk_add_backlog(sk, skb, sk->sk_rcvbuf))) {
		} else if (unlikely(sk_add_backlog(sk, skb,
		sk->sk_rcvbuf + sk->sk_sndbuf))) {
		bh_unlock_sock(sk);
		NET_INC_STATS_BH(net, LINUX_MIB_TCPBACKLOGDROP);
		goto discard_and_relse;

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