tcp: refine pacing rate determination

When TCP pacing was added back in linux-3.12, we chose
to apply a fixed ratio of 200 % against current rate,
to allow probing for optimal throughput even during
slow start phase, where cwnd can be doubled every other gRTT.

At Google, we found it was better applying a different ratio
while in Congestion Avoidance phase.
This ratio was set to 120 %.

We've used the normal tcp_in_slow_start() helper for a while,
then tuned the condition to select the conservative ratio
as soon as cwnd >= ssthresh/2 :

- After cwnd reduction, it is safer to ramp up more slowly,
  as we approach optimal cwnd.
- Initial ramp up (ssthresh == INFINITY) still allows doubling
  cwnd every other RTT.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 0 deletions

diff --git a/Documentation/networking/ip-sysctl.txt b/Documentation/networking/ip-sysctl.txt
index 46e88ed7f41d..ac77a13d2ea2 100644
--- a/Documentation/networking/ip-sysctl.txt
+++ b/Documentation/networking/ip-sysctl.txt
@@ -586,6 +586,21 @@ tcp_min_tso_segs - INTEGER
 	if available window is too small.
 	Default: 2
 
+tcp_pacing_ss_ratio - INTEGER
+	sk->sk_pacing_rate is set by TCP stack using a ratio applied
+	to current rate. (current_rate = cwnd * mss / srtt)
+	If TCP is in slow start, tcp_pacing_ss_ratio is applied
+	to let TCP probe for bigger speeds, assuming cwnd can be
+	doubled every other RTT.
+	Default: 200
+
+tcp_pacing_ca_ratio - INTEGER
+	sk->sk_pacing_rate is set by TCP stack using a ratio applied
+	to current rate. (current_rate = cwnd * mss / srtt)
+	If TCP is in congestion avoidance phase, tcp_pacing_ca_ratio
+	is applied to conservatively probe for bigger throughput.
+	Default: 120
+
 tcp_tso_win_divisor - INTEGER
 	This allows control over what percentage of the congestion window
 	can be consumed by a single TSO frame.