net: fix excessive window updates

Backported from FreeBSD r242252.

Improves netperf by about 10%.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>

bsd/sys/netinet/tcp_output.cc

@@ -509,19 +509,38 @@ after_sack_rexmit:
 	}
 
 	/*
-	 * Compare available window to amount of window
-	 * known to peer (as advertised window less
-	 * next expected input).  If the difference is at least two
-	 * max size segments, or at least 50% of the maximum possible
-	 * window, then want to send a window update to peer.
-	 * Skip this if the connection is in T/TCP half-open state.
-	 * Don't send pure window updates when the peer has closed
-	 * the connection and won't ever send more data.
+	 * Sending of standalone window updates.
+	 *
+	 * Window updates important when we close our window due to a full
+	 * socket buffer and are opening it again after the application
+	 * reads data from it.  Once the window has opened again and the
+	 * remote end starts to send again the ACK clock takes over and
+	 * provides the most current window information.
+	 *
+	 * We must avoid to the silly window syndrome whereas every read
+	 * from the receive buffer, no matter how small, causes a window
+	 * update to be sent.  We also should avoid sending a flurry of
+	 * window updates when the socket buffer had queued a lot of data
+	 * and the application is doing small reads.
+	 *
+	 * Prevent a flurry of pointless window updates by only sending
+	 * an update when we can increase the advertized window by more
+	 * than 1/4th of the socket buffer capacity.  When the buffer is
+	 * getting full or is very small be more aggressive and send an
+	 * update whenever we can increase by two mss sized segments.
+	 * In all other situations the ACK's to new incoming data will
+	 * carry further window increases.
+	 *
+	 * Don't send an independent window update if a delayed
+	 * ACK is pending (it will get piggy-backed on it) or the
+	 * remote side already has done a half-close and won't send
+	 * more data.  Skip this if the connection is in T/TCP
+	 * half-open state.
 	 */
 	if (recwin > 0 && !(tp->t_flags & TF_NEEDSYN) &&
 	    !TCPS_HAVERCVDFIN(tp->t_state)) {
 		/*
-		 * "adv" is the amount we can increase the window,
+		 * "adv" is the amount we could increase the window,
 		 * taking into account that we are limited by
 		 * TCP_MAXWIN << tp->rcv_scale.
 		 */
@@ -541,9 +560,11 @@ after_sack_rexmit:
 		 */
 		if (oldwin >> tp->rcv_scale == (adv + oldwin) >> tp->rcv_scale)
 			goto dontupdate;
-		if (adv >= (long) (2 * tp->t_maxseg))
-			goto send;
-		if (2 * adv >= (long) so->so_rcv.sb_hiwat)
+
+		if (adv >= (long)(2 * tp->t_maxseg) &&
+		    (adv >= (long)(so->so_rcv.sb_hiwat / 4) ||
+		     recwin <= (long)(so->so_rcv.sb_hiwat / 8) ||
+		     so->so_rcv.sb_hiwat <= 8 * tp->t_maxseg))
 			goto send;
 	}
 dontupdate: