--- a/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp	Wed Jun 05 08:59:17 2019 +0200
+++ b/src/hotspot/share/gc/shenandoah/shenandoahArguments.cpp	Wed Jun 05 09:23:09 2019 +0200
@@ -67,12 +67,34 @@
     FLAG_SET_DEFAULT(UseNUMAInterleaving, true);
   }
 
-  FLAG_SET_DEFAULT(ParallelGCThreads,
-                   WorkerPolicy::parallel_worker_threads());
+  // Set up default number of concurrent threads. We want to have cycles complete fast
+  // enough, but we also do not want to steal too much CPU from the concurrently running
+  // application. Using 1/4 of available threads for concurrent GC seems a good
+  // compromise here.
+  if (FLAG_IS_DEFAULT(ConcGCThreads)) {
+    FLAG_SET_DEFAULT(ConcGCThreads, MAX2(1, os::processor_count() / 4));
+  }
+
+  if (ConcGCThreads == 0) {
+    vm_exit_during_initialization("Shenandoah expects ConcGCThreads > 0, check -XX:ConcGCThreads=#");
+  }
 
-  if (FLAG_IS_DEFAULT(ConcGCThreads)) {
-    uint conc_threads = MAX2((uint) 1, ParallelGCThreads);
-    FLAG_SET_DEFAULT(ConcGCThreads, conc_threads);
+  // Set up default number of parallel threads. We want to have decent pauses performance
+  // which would use parallel threads, but we also do not want to do too many threads
+  // that will overwhelm the OS scheduler. Using 1/2 of available threads seems to be a fair
+  // compromise here. Due to implementation constraints, it should not be lower than
+  // the number of concurrent threads.
+  if (FLAG_IS_DEFAULT(ParallelGCThreads)) {
+    FLAG_SET_DEFAULT(ParallelGCThreads, MAX2(1, os::processor_count() / 2));
+  }
+
+  if (ParallelGCThreads == 0) {
+    vm_exit_during_initialization("Shenandoah expects ParallelGCThreads > 0, check -XX:ParallelGCThreads=#");
+  }
+
+  if (ParallelGCThreads < ConcGCThreads) {
+    warning("Shenandoah expects ConcGCThreads <= ParallelGCThreads, adjusting ParallelGCThreads automatically");
+    FLAG_SET_DEFAULT(ParallelGCThreads, ConcGCThreads);
   }
 
   if (FLAG_IS_DEFAULT(ParallelRefProcEnabled)) {

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/gc/shenandoah/options/TestThreadCounts.java	Wed Jun 05 09:23:09 2019 +0200
@@ -0,0 +1,71 @@
+/*
+ * Copyright (c) 2016, 2018, Red Hat, Inc. All rights reserved.
+ *
+ * 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 TestThreadCounts
+ * @summary Test that Shenandoah GC thread counts are handled well
+ * @key gc
+ * @requires vm.gc.Shenandoah & !vm.graal.enabled
+ * @library /test/lib
+ * @modules java.base/jdk.internal.misc
+ *          java.management
+ * @run driver TestThreadCounts
+ */
+
+import jdk.test.lib.process.ProcessTools;
+import jdk.test.lib.process.OutputAnalyzer;
+
+public class TestThreadCounts {
+    public static void main(String[] args) throws Exception {
+        for (int conc = 0; conc < 16; conc++) {
+            for (int par = 0; par < 16; par++) {
+                testWith(conc, par);
+            }
+        }
+    }
+
+    private static void testWith(int conc, int par) throws Exception {
+        ProcessBuilder pb = ProcessTools.createJavaProcessBuilder("-XX:+UnlockDiagnosticVMOptions",
+                "-XX:+UnlockExperimentalVMOptions",
+                "-XX:+UseShenandoahGC",
+                "-XX:ConcGCThreads=" + conc,
+                "-XX:ParallelGCThreads=" + par,
+                "-version");
+        OutputAnalyzer output = new OutputAnalyzer(pb.start());
+
+        if (conc == 0) {
+            output.shouldContain("Shenandoah expects ConcGCThreads > 0");
+            output.shouldHaveExitValue(1);
+        } else if (par == 0) {
+            output.shouldContain("Shenandoah expects ParallelGCThreads > 0");
+            output.shouldHaveExitValue(1);
+        } else if (conc > par) {
+            output.shouldContain("Shenandoah expects ConcGCThreads <= ParallelGCThreads");
+            output.shouldHaveExitValue(0);
+        } else {
+            output.shouldNotContain("Shenandoah expects ConcGCThreads <= ParallelGCThreads");
+            output.shouldHaveExitValue(0);
+        }
+    }
+
+}