--- a/src/hotspot/share/gc/g1/g1Arguments.cpp	Tue Jun 19 09:08:39 2018 +0200
+++ b/src/hotspot/share/gc/g1/g1Arguments.cpp	Tue Jun 19 09:34:39 2018 +0200
@@ -122,6 +122,10 @@
     FLAG_SET_DEFAULT(GCPauseIntervalMillis, MaxGCPauseMillis + 1);
   }
 
+  if (FLAG_IS_DEFAULT(ParallelRefProcEnabled) && ParallelGCThreads > 1) {
+    FLAG_SET_DEFAULT(ParallelRefProcEnabled, true);
+  }
+
   log_trace(gc)("MarkStackSize: %uk  MarkStackSizeMax: %uk", (unsigned int) (MarkStackSize / K), (uint) (MarkStackSizeMax / K));
 
   // By default do not let the target stack size to be more than 1/4 of the entries

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/gc/arguments/TestParallelRefProc.java	Tue Jun 19 09:34:39 2018 +0200
@@ -0,0 +1,74 @@
+/*
+ * Copyright (c) 2018, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test TestParallelRefProc
+ * @key gc
+ * @summary Test defaults processing for -XX:+ParallelRefProcEnabled.
+ * @library /test/lib
+ * @run driver TestParallelRefProc
+ */
+
+import java.util.Arrays;
+import java.util.ArrayList;
+
+import jdk.test.lib.process.OutputAnalyzer;
+import jdk.test.lib.process.ProcessTools;
+
+public class TestParallelRefProc {
+
+    public static void main(String args[]) throws Exception {
+        testFlag(new String[] { "-XX:+UseSerialGC" }, false);
+        testFlag(new String[] { "-XX:+UseConcMarkSweepGC" }, false);
+        testFlag(new String[] { "-XX:+UseParallelGC" }, false);
+        testFlag(new String[] { "-XX:+UseG1GC", "-XX:ParallelGCThreads=1" }, false);
+        testFlag(new String[] { "-XX:+UseG1GC", "-XX:ParallelGCThreads=2" }, true);
+        testFlag(new String[] { "-XX:+UseG1GC", "-XX:-ParallelRefProcEnabled", "-XX:ParallelGCThreads=2" }, false);
+    }
+
+    private static final String parallelRefProcEnabledPattern =
+        " *bool +ParallelRefProcEnabled *= *true +\\{product\\}";
+
+    private static final String parallelRefProcDisabledPattern =
+        " *bool +ParallelRefProcEnabled *= *false +\\{product\\}";
+
+    private static void testFlag(String[] args, boolean expectedTrue) throws Exception {
+        ArrayList<String> result = new ArrayList<String>();
+        result.addAll(Arrays.asList(args));
+        result.add("-XX:+PrintFlagsFinal");
+        result.add("-version");
+        ProcessBuilder pb = ProcessTools.createJavaProcessBuilder(result.toArray(new String[0]));
+
+        OutputAnalyzer output = new OutputAnalyzer(pb.start());
+
+        output.shouldHaveExitValue(0);
+
+        final String expectedPattern = expectedTrue ? parallelRefProcEnabledPattern : parallelRefProcDisabledPattern;
+
+        String value = output.firstMatch(expectedPattern);
+        if (value == null) {
+            throw new RuntimeException(
+                Arrays.toString(args) + " didn't set ParallelRefProcEnabled to " + expectedTrue + " as expected");
+        }
+    }
+}