From 42eef7a0bb0989cd50d74e673422ff98a0ce4d7b Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Thu, 4 Dec 2014 16:13:49 -0800 Subject: [PATCH] tcp_cubic: refine Hystart delay threshold In commit 2b4636a5f8ca ("tcp_cubic: make the delay threshold of HyStart less sensitive"), HYSTART_DELAY_MIN was changed to 4 ms. The remaining problem is that using delay_min + (delay_min/16) as the threshold is too sensitive. 6.25 % of variation is too small for rtt above 60 ms, which are not uncommon. Lets use 12.5 % instead (delay_min + (delay_min/8)) Tested: 80 ms RTT between peers, FQ/pacing packet scheduler on sender. 10 bulk transfers of 10 seconds : nstat >/dev/null for i in `seq 1 10` do netperf -H remote -- -k THROUGHPUT | grep THROUGHPUT done nstat | grep Hystart With the 6.25 % threshold : THROUGHPUT=20.66 THROUGHPUT=249.38 THROUGHPUT=254.10 THROUGHPUT=14.94 THROUGHPUT=251.92 THROUGHPUT=237.73 THROUGHPUT=19.18 THROUGHPUT=252.89 THROUGHPUT=21.32 THROUGHPUT=15.58 TcpExtTCPHystartTrainDetect 2 0.0 TcpExtTCPHystartTrainCwnd 4756 0.0 TcpExtTCPHystartDelayDetect 5 0.0 TcpExtTCPHystartDelayCwnd 180 0.0 With the 12.5 % threshold THROUGHPUT=251.09 THROUGHPUT=247.46 THROUGHPUT=250.92 THROUGHPUT=248.91 THROUGHPUT=250.88 THROUGHPUT=249.84 THROUGHPUT=250.51 THROUGHPUT=254.15 THROUGHPUT=250.62 THROUGHPUT=250.89 TcpExtTCPHystartTrainDetect 1 0.0 TcpExtTCPHystartTrainCwnd 3175 0.0 Signed-off-by: Eric Dumazet Acked-by: Neal Cardwell Tested-by: Neal Cardwell Signed-off-by: David S. Miller --- net/ipv4/tcp_cubic.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c index c1d07c7ed03d..6b6002416a73 100644 --- a/net/ipv4/tcp_cubic.c +++ b/net/ipv4/tcp_cubic.c @@ -393,7 +393,7 @@ static void hystart_update(struct sock *sk, u32 delay) ca->sample_cnt++; } else { if (ca->curr_rtt > ca->delay_min + - HYSTART_DELAY_THRESH(ca->delay_min>>4)) { + HYSTART_DELAY_THRESH(ca->delay_min >> 3)) { ca->found |= HYSTART_DELAY; NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPHYSTARTDELAYDETECT); -- 2.20.1