8274349: ForkJoinPool.commonPool() does not work with 1 CPU

Co-authored-by: Doug Lea <dl@openjdk.org>
Reviewed-by: shade, martin

src/java.base/share/classes/java/util/concurrent/ForkJoinPool.java

@@ -2561,7 +2561,7 @@ public class ForkJoinPool extends AbstractExecutorService {
      * overridden by system properties
      */
     private ForkJoinPool(byte forCommonPoolOnly) {
-        int parallelism = Runtime.getRuntime().availableProcessors() - 1;
+        int parallelism = Math.max(1, Runtime.getRuntime().availableProcessors() - 1);
         ForkJoinWorkerThreadFactory fac = null;
         UncaughtExceptionHandler handler = null;
         try {  // ignore exceptions in accessing/parsing properties

test/jdk/java/util/concurrent/forkjoin/Uniprocessor.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2021, 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
+ * @bug 8274349
+ * @run main/othervm -XX:ActiveProcessorCount=1 Uniprocessor
+ * @summary Check the default FJ pool has a reasonable default parallelism
+ *          level in a uniprocessor environment.
+ */
+
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ForkJoinPool;
+
+public class Uniprocessor {
+
+    static volatile boolean done = false;
+
+    public static void main(String[] args) throws InterruptedException {
+        // If the default parallelism were zero then this task would not
+        // complete and the test will timeout.
+        CountDownLatch ran = new CountDownLatch(1);
+        ForkJoinPool.commonPool().submit(() -> ran.countDown());
+        ran.await();
+    }
+}

test/jdk/java/util/concurrent/tck/ForkJoinPoolTest.java

@@ -51,6 +51,7 @@ import java.util.concurrent.ForkJoinWorkerThread;
 import java.util.concurrent.Future;
 import java.util.concurrent.RecursiveTask;
 import java.util.concurrent.RejectedExecutionException;
+import java.util.concurrent.ThreadLocalRandom;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.locks.ReentrantLock;
@@ -223,13 +224,46 @@ public class ForkJoinPoolTest extends JSR166TestCase {
     /**
      * getParallelism returns size set in constructor
      */
-    public void testGetParallelism() {
-        ForkJoinPool p = new ForkJoinPool(1);
+    public void testGetParallelism_requestedValue() {
+        int parallelism = ThreadLocalRandom.current().nextInt(1, 4);
+        ForkJoinPool p = new ForkJoinPool(parallelism);
         try (PoolCleaner cleaner = cleaner(p)) {
-            assertEquals(1, p.getParallelism());
+            assertEquals(parallelism, p.getParallelism());
         }
     }
 
+    private static int availableProcessors() {
+        return Runtime.getRuntime().availableProcessors();
+    }
+
+    /**
+     * default pool parallelism is availableProcessors()
+     */
+    public void testParallelism_defaultValue() {
+        ForkJoinPool p = new ForkJoinPool();
+        try (PoolCleaner cleaner = cleaner(p)) {
+            assertEquals(availableProcessors(), p.getParallelism());
+        }
+    }
+
+    /**
+     * default common pool parallelism is max(1, availableProcessors() - 1)
+     * But getParallelism() returns 1 when property-requested parallelism is 0.
+     */
+    public void testCommonPoolParallelism_defaultValue() {
+        if (!testImplementationDetails) return;
+
+        Integer propertyParallelism =
+            Integer.getInteger(
+                "java.util.concurrent.ForkJoinPool.common.parallelism");
+
+        int expectedParallelism = (propertyParallelism == null)
+            ? Math.max(1, availableProcessors() - 1)
+            : Math.max(1, propertyParallelism);
+        assertEquals(expectedParallelism,
+                     ForkJoinPool.commonPool().getParallelism());
+    }
+
     /**
      * getPoolSize returns number of started workers.
      */