From 83eecff731c1a049b12f01fb699fa15ab7ddac2e Mon Sep 17 00:00:00 2001
From: Evan Hunt <each@isc.org>
Date: Wed, 19 Feb 2014 21:54:43 -0800
Subject: [PATCH] [master] adjust default -U

3751.	[tuning]	The default setting for the -U option (setting
			the number of UDP listeners per interface) has
			been adjusted to improve performance. [RT #35417]
---
 CHANGES                 |  4 ++++
 bin/named/main.c        | 10 ++++++++--
 bin/named/named.docbook | 10 ++++++----
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/CHANGES b/CHANGES
index bd3faf5184..06a30c54b0 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,5 +1,9 @@
 	--- 9.10.0b1 released ---
 
+3751.	[tuning]	The default setting for the -U option (setting
+			the number of UDP listeners per interface) has
+			been adjusted to improve performance. [RT #35417]
+
 3750.	[experimental]	Partially implement EDNS EXPIRE option as described
 			in draft-andrews-dnsext-expire-00.  Retrivial of
 			remaining time to expiry from slave zones is supported.
diff --git a/bin/named/main.c b/bin/named/main.c
index f9eaf7c281..b1951efb39 100644
--- a/bin/named/main.c
+++ b/bin/named/main.c
@@ -646,8 +646,14 @@ create_managers(void) {
 #ifdef WIN32
 	ns_g_udpdisp = 1;
 #else
-	if (ns_g_udpdisp == 0)
-		ns_g_udpdisp = ns_g_cpus_detected;
+	if (ns_g_udpdisp == 0) {
+		if (ns_g_cpus_detected == 1)
+			ns_g_udpdisp = 1;
+		else if (ns_g_cpus_detected < 4)
+			ns_g_udpdisp = 2;
+		else
+			ns_g_udpdisp = ns_g_cpus_detected / 2;
+	}
 	if (ns_g_udpdisp > ns_g_cpus)
 		ns_g_udpdisp = ns_g_cpus;
 #endif
diff --git a/bin/named/named.docbook b/bin/named/named.docbook
index f70787b142..307be93fe8 100644
--- a/bin/named/named.docbook
+++ b/bin/named/named.docbook
@@ -314,10 +314,12 @@
             Use <replaceable class="parameter">#listeners</replaceable>
             worker threads to listen for incoming UDP packets on each
             address.  If not specified, <command>named</command> will
-            use the number of detected CPUs.  If <option>-n</option>
-            has been set to a higher value than the number of CPUs,
-            then <option>-U</option> may be increased as high as that
-            value, but no higher.
+            calculate a default value based on the number of detected
+            CPUs: 1 for 1 CPU, 2 for 2-4 CPUs, and the number of
+            detected CPUs divided by 2 for values higher than 4.
+            If <option>-n</option> has been set to a higher value than
+            the number of detected CPUs, then <option>-U</option> may
+            be increased as high as that value, but no higher.
           </para>
         </listitem>
       </varlistentry>