7903367: JMH: Add JMHSample_39_MemoryAccess

Reviewed-by: shade

jmh-samples/src/main/java/org/openjdk/jmh/samples/JMHSample_39_MemoryAccess.java

@@ -0,0 +1,135 @@
+/*
+ * Copyright (c) 2015, Oracle America, Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ *  * Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ *
+ *  * Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ *  * Neither the name of Oracle nor the names of its contributors may be used
+ *    to endorse or promote products derived from this software without
+ *    specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
+ * THE POSSIBILITY OF SUCH DAMAGE.
+ */
+package org.openjdk.jmh.samples;
+
+import org.openjdk.jmh.annotations.*;
+import org.openjdk.jmh.runner.Runner;
+import org.openjdk.jmh.runner.RunnerException;
+import org.openjdk.jmh.runner.options.Options;
+import org.openjdk.jmh.runner.options.OptionsBuilder;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.List;
+import java.util.Random;
+import java.util.concurrent.TimeUnit;
+
+@BenchmarkMode(Mode.AverageTime)
+@OutputTimeUnit(TimeUnit.MILLISECONDS)
+@Warmup(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
+@Measurement(iterations = 5, time = 1, timeUnit = TimeUnit.SECONDS)
+@Fork(5)
+@State(Scope.Benchmark)
+public class JMHSample_39_MemoryAccess {
+    public static final int N = 20_000_000;
+
+    /*
+     * This example highlights the pitfall of accidentally measuring memory access instead of processing time.
+     *
+     * An int array has got a different memory layout than an ArrayList of boxed ints.
+     * This can lead to useless results because the memory access is completely different.
+     * Arrays save all their ints in one block on the heap while ArrayLists don't.
+     * They save only references to the boxed ints in one block.
+     * All the references point to the boxed ints which are usually spread all over the heap.
+     * This leads to many cache misses with a big error:
+     *
+     * Benchmark                                       Mode  Cnt    Score   Error  Units
+     * JMHSample_39_MemoryAccess.sumArray              avgt   25    4.887 ± 0.008  ms/op
+     * JMHSample_39_MemoryAccess.sumArrayList          avgt   25   35.765 ± 0.112  ms/op
+     * JMHSample_39_MemoryAccess.sumArrayListShuffled  avgt   25  169.301 ± 1.064  ms/op
+     *
+     * The Java Object Layout (JOL) is a tool with which the different memory layouts of arrays and ArrayLists can be
+     * examined in more detail.
+     */
+
+    private int[] intArray = new int[N];
+    private List<Integer> intList = new ArrayList<>(N);
+    private List<Integer> shuffledIntList = new ArrayList<>(N);
+
+    @Setup
+    public void setup() {
+        Random random = new Random(1234);
+        for (int i = 0; i < N; i++) {
+            intArray[i] = random.nextInt();
+            intList.add(intArray[i]);
+            shuffledIntList.add(intArray[i]);
+        }
+        Collections.shuffle(shuffledIntList);
+    }
+
+    @Benchmark
+    public long sumArray() {
+        long sum = 0;
+        for (int i = 0; i < N; i++) {
+            sum += intArray[i];
+        }
+        return sum;
+    }
+
+    @Benchmark
+    public long sumArrayList() {
+        long sum = 0;
+        for (int i = 0; i < N; i++) {
+            sum += intList.get(i);
+        }
+        return sum;
+    }
+
+    @Benchmark
+    public long sumArrayListShuffled() {
+        long sum = 0;
+        for (int i = 0; i < N; i++) {
+            sum += shuffledIntList.get(i);
+        }
+        return sum;
+    }
+
+    /*
+     * ============================== HOW TO RUN THIS TEST: ====================================
+     *
+     * You can run this test:
+     *
+     * a) Via the command line:
+     *    $ mvn clean install
+     *    $ java -jar target/benchmarks.jar JMHSample_39
+     *
+     * b) Via the Java API:
+     *    (see the JMH homepage for possible caveats when running from IDE:
+     *      http://openjdk.java.net/projects/code-tools/jmh/)
+     */
+    public static void main(String[] args) throws RunnerException {
+        Options opt = new OptionsBuilder()
+                .include(".*" + JMHSample_39_MemoryAccess.class.getSimpleName() + ".*")
+                .build();
+
+        new Runner(opt).run();
+    }
+}