Commit d578e18c authored Mar 10, 2015 by Neal Cardwell Committed by David S. Miller Mar 11, 2015

tcp: restore 1.5x per RTT limit to CUBIC cwnd growth in congestion avoidance



Commit 814d488c ("tcp: fix the timid additive increase on stretch
ACKs") fixed a bug where tcp_cong_avoid_ai() would either credit a
connection with an increase of snd_cwnd_cnt, or increase snd_cwnd, but
not both, resulting in cwnd increasing by 1 packet on at most every
alternate invocation of tcp_cong_avoid_ai().

Although the commit correctly implemented the CUBIC algorithm, which
can increase cwnd by as much as 1 packet per 1 packet ACKed (2x per
RTT), in practice that could be too aggressive: in tests on network
paths with small buffers, YouTube server retransmission rates nearly
doubled.

This commit restores CUBIC to a maximum cwnd growth rate of 1 packet
per 2 packets ACKed (1.5x per RTT). In YouTube tests this restored
retransmit rates to low levels.

Testing: This patch has been tested in datacenter netperf transfers
and live youtube.com and google.com servers.

Fixes: 9cd981dc ("tcp: fix stretch ACK bugs in CUBIC")
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent 9949afa4

net/ipv4/tcp_cubic.c

+4 −2

Original line number	Diff line number	Diff line
		@@ -306,8 +306,10 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)
		}
		}

		if (ca->cnt == 0) /* cannot be zero */
		ca->cnt = 1;
		/* The maximum rate of cwnd increase CUBIC allows is 1 packet per
		* 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.
		*/
		ca->cnt = max(ca->cnt, 2U);
		}

		static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)