Subject: [PATCH 7/7] tcp_cubic: fix low utilization of CUBIC with HyStart

From: Stephen Hemminger <[email-masked]>
Date: Mon, 14 Mar 2011 10:52:18 -0700

From: Sangtae Ha <[email-masked]>

HyStart sets the initial exit point of slow start.
Suppose that HyStart exits at 0.5BDP in a BDP network and no history exists.
If the BDP of a network is large, CUBIC's initial cwnd growth may be
too conservative to utilize the link.
CUBIC increases the cwnd 20% per RTT in this case.

Signed-off-by: Sangtae Ha <[email-masked]>
Acked-by: Stephen Hemminger <[email-masked]>
---
 net/ipv4/tcp_cubic.c |    9 +++++++++
 1 files changed, 9 insertions(+), 0 deletions(-)

--- a/net/ipv4/tcp_cubic.c	2011-03-14 08:32:46.347993869 -0700
+++ b/net/ipv4/tcp_cubic.c	2011-03-14 10:57:00.846549449 -0700
@@ -270,6 +270,13 @@ static inline void bictcp_update(struct
 		ca->cnt = 100 * cwnd;              /* very small increment*/
 	}
 
+	/*
+	 * The initial growth of cubic function may be too conservative
+	 * when the available bandwidth is still unknown.
+	 */
+	if (ca->loss_cwnd == 0 && ca->cnt > 20)
+		ca->cnt = 20;	/* increase cwnd 5% per RTT */
+
 	/* TCP Friendly */
 	if (tcp_friendliness) {
 		u32 scale = beta_scale;