netdev - [PATCH net-next 2/2] tcp_cubic: refine Hystart delay threshold

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Message-ID: <1417738429.22424.29.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 04 Dec 2014 16:13:49 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev <netdev@...r.kernel.org>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Sangtae Ha <sangtae.ha@...il.com>
Subject: [PATCH net-next 2/2] tcp_cubic: refine Hystart delay threshold

From: Eric Dumazet <edumazet@...gle.com>

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 <edumazet@...gle.com>
---
 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 c1d07c7ed03d7d37fa28d1509093e686f78134d2..6b6002416a73950d493661ea1459870f49917efc 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);


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