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

nealcardwell · davem330 · commit d578e18ce93f · 2015-03-11T16:51:51.000-04:00
Commit 814d488 ("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: 9cd981d ("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>
diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
@@ -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)

Original file line number	Diff line number	Diff line change
`@@ -306,8 +306,10 @@ static inline void bictcp_update(struct bictcp *ca, u32 cwnd, u32 acked)`
`306`	`306`	`}`
`307`	`307`	`}`
`308`	`308`
`309`		`- if (ca->cnt == 0) /* cannot be zero */`
`310`		`- ca->cnt = 1;`
	`309`	`+ /* The maximum rate of cwnd increase CUBIC allows is 1 packet per`
	`310`	`+ * 2 packets ACKed, meaning cwnd grows at 1.5x per RTT.`
	`311`	`+ */`
	`312`	`+ ca->cnt = max(ca->cnt, 2U);`
`311`	`313`	`}`
`312`	`314`
`313`	`315`	`static void bictcp_cong_avoid(struct sock *sk, u32 ack, u32 acked)`