Change TCPTV_MIN to be independent of HZ. While it was documented to

be in ticks "for algorithm stability" when originally committed, it turns
out that it has a significant impact in timing out connections.  When we
changed HZ from 100 to 1000, this had a big effect on reducing the time
before dropping connections.

To demonstrate, boot with kern.hz=100.  ssh to a box on local ethernet
and establish a reliable round-trip-time (ie: type a few commands).
Then unplug the ethernet and press a key.  Time how long it takes to
drop the connection.

The old behavior (with hz=100) caused the connection to typically drop
between 90 and 110 seconds of getting no response.

Now boot with kern.hz=1000 (default).  The same test causes the ssh session
to drop after just 9-10 seconds.  This is a big deal on a wifi connection.

With kern.hz=1000, change sysctl net.inet.tcp.rexmit_min from 3 to 30.
Note how it behaves the same as when HZ was 100.  Also, note that when
booting with hz=100, net.inet.tcp.rexmit_min *used* to be 30.

This commit changes TCPTV_MIN to be scaled with hz.  rexmit_min should
always be about 30.  If you set hz to Really Slow(TM), there is a safety
feature to prevent a value of 0 being used.

This may be revised in the future, but for the time being, it restores the
old, pre-hz=1000 behavior, which is significantly less annoying.

As a workaround, to avoid rebooting or rebuilding a kernel, you can run
"sysctl net.inet.tcp.rexmit_min=30" and add "net.inet.tcp.rexmit_min=30"
to /etc/sysctl.conf.  This is safe to run from 6.0 onwards.

Approved by:  re (rwatson)
Reviewed by:  andre, silby

1 files changed, 1 insertions, 1 deletions

diff --git a/sys/netinet/tcp_timer.h b/sys/netinet/tcp_timer.h
index 8a8db5d..f4392dd 100644
--- a/sys/netinet/tcp_timer.h
+++ b/sys/netinet/tcp_timer.h
@@ -111,7 +111,7 @@
  * The prior minimum of 1*hz (1 second) badly breaks throughput on any
  * networks faster then a modem that has minor (e.g. 1%) packet loss.
  */
-#define	TCPTV_MIN	( 3 )			/* minimum allowable value */
+#define	TCPTV_MIN	( hz/33 )		/* minimum allowable value */
 #define TCPTV_CPU_VAR	( hz/5 )		/* cpu variance allowed (200ms) */
 #define	TCPTV_REXMTMAX	( 64*hz)		/* max allowable REXMT value */