From: Matthew Dillon <dillon@apollo.backplane.com>
Date: Tue, 10 Nov 2015 00:21:14 +0000 (-0800)
Subject: kernel - Improve tcp starting window
X-Git-Tag: v4.5.0~126
X-Git-Url: https://gitweb.dragonflybsd.org/dragonfly.git/commitdiff_plain/5353fab65372e67d7adbd6c4a78c2479399fff26

kernel - Improve tcp starting window

* The tcp sender inflight management was forcing a starting window that
  was way too small for international connections, causing a bulk fetch
  to take several seconds to ramp up.  This management is enabled by default.

* Add net.inet.tcp.inflight_start to set the starting window and set the
  default to something more reasonable, 33792.  Inflight management previously
  used net.inet.tcp.inflight_min as the starting value, which was 6144 and
  way too small.

* This will significantly improve the performance of small fetches, e.g.
  pkg install's, update's, and upgrades, and the performance of small web
  server fetches occuring over high latency connections.

Reported-by: tuxillo
---

diff --git a/sys/netinet/tcp_subr.c b/sys/netinet/tcp_subr.c
index f7356da5d3..15eae3813c 100644
--- a/sys/netinet/tcp_subr.c
+++ b/sys/netinet/tcp_subr.c
@@ -231,6 +231,17 @@ static int tcp_inflight_debug = 0;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, inflight_debug, CTLFLAG_RW,
     &tcp_inflight_debug, 0, "Debug TCP inflight calculations");
 
+/*
+ * NOTE: tcp_inflight_start is essentially the starting receive window
+ *	 for a connection.  If set too low then fetches over tcp
+ *	 connections will take noticably longer to ramp-up over
+ *	 high-latency connections.  6144 is too low for a default,
+ *	 use something more reasonable.
+ */
+static int tcp_inflight_start = 33792;
+SYSCTL_INT(_net_inet_tcp, OID_AUTO, inflight_start, CTLFLAG_RW,
+    &tcp_inflight_start, 0, "Start value for TCP inflight window");
+
 static int tcp_inflight_min = 6144;
 SYSCTL_INT(_net_inet_tcp, OID_AUTO, inflight_min, CTLFLAG_RW,
     &tcp_inflight_min, 0, "Lower bound for TCP inflight window");
@@ -1961,7 +1972,7 @@ tcp_xmit_bandwidth_limit(struct tcpcb *tp, tcp_seq ack_seq)
 		tp->t_bw_rtttime = save_ticks;
 		tp->t_bw_rtseq = ack_seq;
 		if (tp->snd_bandwidth == 0)
-			tp->snd_bandwidth = tcp_inflight_min;
+			tp->snd_bandwidth = tcp_inflight_start;
 		return;
 	}