diff --git a/CHANGES b/CHANGES
index 81a037d3cc..0325dce907 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,7 @@
+3440.	[performance]	Implement adaptive read-write locks, reducing the
+			overhead of locks that are only held briefly.
+			[RT #37329]
+
 4339.	[test]		Use "mdig" to test pipelined queries. [RT #41929]
 
 4338.	[bug]		Reimplement change 4324 as it wasn't properly doing
diff --git a/configure b/configure
index 5483289f0f..7ce158192d 100755
--- a/configure
+++ b/configure
@@ -712,6 +712,7 @@ DNSTAPOBJS
 DNSTAPSRCS
 DNSTAP
 PROTOC_C
+ISC_PLATFORM_BUSYWAITNOP
 ISC_ARCH_DIR
 ISC_PLATFORM_USEMACASM
 ISC_PLATFORM_USESTDASM
@@ -19964,6 +19965,115 @@ $as_echo "#define HAVE_BUILTIN_EXPECT 1" >>confdefs.h
 
 fi
 
+#
+# CPU relax (for spin locks)
+#
+if $use_threads
+then
+	case "$host" in
+	i[3456]86-*)
+		# x86_32
+		{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if asm(\"rep; nop\"); works" >&5
+$as_echo_n "checking if asm(\"rep; nop\"); works... " >&6; }
+		cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+asm("rep; nop");
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP asm(\"rep; nop\")"
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+		;;
+	x86_64-*|amd64-*)
+		# x86_64
+		{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if asm(\"rep; nop\"); works" >&5
+$as_echo_n "checking if asm(\"rep; nop\"); works... " >&6; }
+		cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+asm("rep; nop");
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP asm(\"rep; nop\")"
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+		;;
+	ia64-*)
+		# ia64
+		{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if asm(\"hint @pause\"); works" >&5
+$as_echo_n "checking if asm(\"hint @pause\"); works... " >&6; }
+		cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+asm("hint @pause");
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP asm(\"hint @pause\")"
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+		;;
+	sparc-*)
+		# sparc
+		{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if cpu_relax(); or __cpu_relax(); works" >&5
+$as_echo_n "checking if cpu_relax(); or __cpu_relax(); works... " >&6; }
+		ac_fn_c_check_func "$LINENO" "cpu_relax" "ac_cv_func_cpu_relax"
+if test "x$ac_cv_func_cpu_relax" = xyes; then :
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP cpu_relax()"
+else
+  ac_fn_c_check_func "$LINENO" "__cpu_relax" "ac_cv_func___cpu_relax"
+if test "x$ac_cv_func___cpu_relax" = xyes; then :
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+$as_echo "yes" >&6; }
+		   ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP __cpu_relax()"
+else
+  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
+$as_echo "no" >&6; }
+fi
+
+fi
+
+		;;
+	esac
+fi
+
+
+
 #
 # Activate "rrset-order fixed" or not?
 #
diff --git a/configure.in b/configure.in
index 506a49e58a..e6a8b3e864 100644
--- a/configure.in
+++ b/configure.in
@@ -3910,6 +3910,58 @@ if test "$have_builtin_expect" = "yes"; then
         AC_DEFINE(HAVE_BUILTIN_EXPECT, 1, [Define to 1 if the compiler supports __builtin_expect.])
 fi
 
+#
+# CPU relax (for spin locks)
+#
+if $use_threads
+then
+	case "$host" in
+	[i[3456]86-*])
+		# x86_32
+		AC_MSG_CHECKING([if asm("rep; nop"); works])
+		AC_TRY_COMPILE(,[asm("rep; nop");],
+		[AC_MSG_RESULT(yes)
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP asm(\"rep; nop\")"],
+		[AC_MSG_RESULT(no)],
+		[AC_MSG_RESULT([cross compile, assume yes])
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP asm(\"rep; nop\")"])
+		;;
+	x86_64-*|amd64-*)
+		# x86_64
+		AC_MSG_CHECKING([if asm("rep; nop"); works])
+		AC_TRY_COMPILE(,[asm("rep; nop");],
+		[AC_MSG_RESULT(yes)
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP asm(\"rep; nop\")"],
+		[AC_MSG_RESULT(no)],
+		[AC_MSG_RESULT([cross compile, assume yes])
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP asm(\"rep; nop\")"])
+		;;
+	ia64-*)
+		# ia64
+		AC_MSG_CHECKING([if asm("hint @pause"); works])
+		AC_TRY_COMPILE(,[asm("hint @pause");],
+		[AC_MSG_RESULT(yes)
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP asm(\"hint @pause\")"],
+		[AC_MSG_RESULT(no)],
+		[AC_MSG_RESULT([cross compile, assume yes])
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP asm(\"hint @pause\")"])
+		;;
+	sparc-*)
+		# sparc
+		AC_MSG_CHECKING([if cpu_relax(); or __cpu_relax(); works])
+		AC_CHECK_FUNC(cpu_relax,
+		[AC_MSG_RESULT(yes)
+		 ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP cpu_relax()"],
+		 [AC_CHECK_FUNC(__cpu_relax,
+		  [AC_MSG_RESULT(yes)
+		   ISC_PLATFORM_BUSYWAITNOP="#define ISC_PLATFORM_BUSYWAITNOP __cpu_relax()"],
+		   [AC_MSG_RESULT(no)])])
+		;;
+	esac
+fi
+
+AC_SUBST(ISC_PLATFORM_BUSYWAITNOP)
+
 #
 # Activate "rrset-order fixed" or not?
 #
diff --git a/lib/isc/include/isc/platform.h.in b/lib/isc/include/isc/platform.h.in
index 75cdec33a3..2b451409e6 100644
--- a/lib/isc/include/isc/platform.h.in
+++ b/lib/isc/include/isc/platform.h.in
@@ -320,6 +320,11 @@
  */
 @ISC_PLATFORM_USESTDASM@
 
+/*
+ * Define with the busy wait nop asm or function call.
+ */
+@ISC_PLATFORM_BUSYWAITNOP@
+
 /*
  * Define if the platform has <strings.h>.
  */
diff --git a/lib/isc/include/isc/rwlock.h b/lib/isc/include/isc/rwlock.h
index 28052cdd7f..9186c8afc6 100644
--- a/lib/isc/include/isc/rwlock.h
+++ b/lib/isc/include/isc/rwlock.h
@@ -44,6 +44,7 @@ struct isc_rwlock {
 	/* Unlocked. */
 	unsigned int		magic;
 	isc_mutex_t		lock;
+	isc_int32_t		spins;
 
 #if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)
 	/*
diff --git a/lib/isc/rwlock.c b/lib/isc/rwlock.c
index 461d6e295a..6e29008865 100644
--- a/lib/isc/rwlock.c
+++ b/lib/isc/rwlock.c
@@ -44,6 +44,15 @@
 #define RWLOCK_DEFAULT_WRITE_QUOTA 4
 #endif
 
+#ifndef RWLOCK_MAX_ADAPTIVE_COUNT
+#define RWLOCK_MAX_ADAPTIVE_COUNT 100
+#endif
+
+#if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)
+static isc_result_t
+isc__rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type);
+#endif
+
 #ifdef ISC_RWLOCK_TRACE
 #include <stdio.h>		/* Required for fprintf/stderr. */
 #include <isc/thread.h>		/* Required for isc_thread_self(). */
@@ -85,6 +94,7 @@ isc_rwlock_init(isc_rwlock_t *rwl, unsigned int read_quota,
 	 */
 	rwl->magic = 0;
 
+	rwl->spins = 0;
 #if defined(ISC_PLATFORM_HAVEXADD) && defined(ISC_PLATFORM_HAVECMPXCHG)
 	rwl->write_requests = 0;
 	rwl->write_completions = 0;
@@ -238,8 +248,8 @@ isc_rwlock_destroy(isc_rwlock_t *rwl) {
 #define WRITER_ACTIVE	0x1
 #define READER_INCR	0x2
 
-isc_result_t
-isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+static isc_result_t
+isc__rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	isc_int32_t cntflag;
 
 	REQUIRE(VALID_RWLOCK(rwl));
@@ -348,6 +358,30 @@ isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	return (ISC_R_SUCCESS);
 }
 
+isc_result_t
+isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
+	isc_int32_t cnt = 0;
+	isc_int32_t max_cnt = rwl->spins * 2 + 10;
+	isc_result_t result = ISC_R_SUCCESS;
+
+	if (max_cnt > RWLOCK_MAX_ADAPTIVE_COUNT)
+		max_cnt = RWLOCK_MAX_ADAPTIVE_COUNT;
+
+	do {
+		if (cnt++ >= max_cnt) {
+			result = isc__rwlock_lock(rwl, type);
+			break;
+		}
+#ifdef ISC_PLATFORM_BUSYWAITNOP
+		ISC_PLATFORM_BUSYWAITNOP;
+#endif
+	} while (isc_rwlock_trylock(rwl, type) != ISC_R_SUCCESS);
+
+	rwl->spins += (cnt - rwl->spins) / 8;
+
+	return (result);
+}
+
 isc_result_t
 isc_rwlock_trylock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
 	isc_int32_t cntflag;
@@ -606,7 +640,26 @@ doit(isc_rwlock_t *rwl, isc_rwlocktype_t type, isc_boolean_t nonblock) {
 
 isc_result_t
 isc_rwlock_lock(isc_rwlock_t *rwl, isc_rwlocktype_t type) {
-	return (doit(rwl, type, ISC_FALSE));
+	isc_int32_t cnt = 0;
+	isc_int32_t max_cnt = rwl->spins * 2 + 10;
+	isc_result_t result = ISC_R_SUCCESS;
+
+	if (max_cnt > RWLOCK_MAX_ADAPTIVE_COUNT)
+		max_cnt = RWLOCK_MAX_ADAPTIVE_COUNT;
+
+	do {
+		if (cnt++ >= max_cnt) {
+			result = doit(rwl, type, ISC_FALSE);
+			break;
+		}
+#ifdef ISC_PLATFORM_BUSYWAITNOP
+		ISC_PLATFORM_BUSYWAITNOP;
+#endif
+	} while (doit(rwl, type, ISC_TRUE) != ISC_R_SUCCESS);
+
+	rwl->spins += (cnt - rwl->spins) / 8;
+
+	return (ISC_R_SUCCESS);
 }
 
 isc_result_t
diff --git a/lib/isc/win32/include/isc/platform.h.in b/lib/isc/win32/include/isc/platform.h.in
index bbd35aca93..12a2b4f78c 100644
--- a/lib/isc/win32/include/isc/platform.h.in
+++ b/lib/isc/win32/include/isc/platform.h.in
@@ -109,6 +109,11 @@
  */
 @ISC_PLATFORM_HAVECMPXCHG@
 
+/*
+ * Define with the busy wait nop asm or function call.
+ */
+@ISC_PLATFORM_BUSYWAITNOP@
+
 /*
  * If the strcasestr() operation is not available on this platform,
  * ISC_PLATFORM_NEEDSTRCASESTR will be defined.
diff --git a/win32utils/Configure b/win32utils/Configure
index 28899ef20f..79ae31e31e 100644
--- a/win32utils/Configure
+++ b/win32utils/Configure
@@ -383,7 +383,8 @@ my @substdefh = ("AES_CC",
 
 my %configdefp;
 
-my @substdefp = ("ISC_PLATFORM_HAVEATOMICSTORE",
+my @substdefp = ("ISC_PLATFORM_BUSYWAITNOP",
+                 "ISC_PLATFORM_HAVEATOMICSTORE",
                  "ISC_PLATFORM_HAVEATOMICSTOREQ",
                  "ISC_PLATFORM_HAVECMPXCHG",
                  "ISC_PLATFORM_HAVEXADD",
@@ -693,11 +694,13 @@ if (($want_win32 eq "yes") && ($want_x64 eq "yes")) {
     $configvar{"BUILD_PLATFORM"} = "Win32";
     $configvar{"MACHINE"} = "/machine:X86";
     $configvar{"BUILD_MACHINE"} = "/machine:X86";
+    $configdefp{"ISC_PLATFORM_BUSYWAITNOP"} = "__asm { rep nop }";
 } elsif ($want_x64 eq "yes") {
     $configvar{"PLATFORM"} = "x64";
     $configvar{"BUILD_PLATFORM"} = "x64";
     $configvar{"MACHINE"} = "/machine:X64";
     $configvar{"BUILD_MACHINE"} = "/machine:X64";
+    $configdefp{"ISC_PLATFORM_BUSYWAITNOP"} = "__asm { rep nop }";
 }
 
 # get the version information