From 43e122b014c955a33220fabbd09c4b5e4f422c3c Mon Sep 17 00:00:00 2001 From: Eric Dumazet <edumazet@google.com> Date: Fri, 21 Aug 2015 17:38:02 -0700 Subject: 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> --- Documentation/networking/ip-sysctl.txt | 15 +++++++++++++++ 1 file changed, 15 insertions(+) (limited to 'Documentation') 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. -- cgit v1.2.3