tcp_cubic: fix low utilization of CUBIC with HyStart

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 <sangtae.ha@gmail.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 7 insertions, 0 deletions

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 7172c129ff19..90d92dd4cf13 100644
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -270,6 +270,13 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd)
                 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;