test/MemoryRamp.java: demonstrate slow multithreaded memory access.

test/MemoryRamp.java

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+/**
+ * Demonstrates slow multithreaded memory access.
+ */
+public class MemoryRamp implements Runnable {
+
+    private static final int ARRAY_SIZE = 10 * 1024 * 1024; //10 MB in byte
+    private static final boolean ACCESS_ARRAY = true;
+
+    @Override
+    public void run() {
+        mem();
+    }
+
+    public static void main(String[] args) {
+        // get timing for single thread
+        long singleTime = mem();
+        System.out.println("Single thread: " + singleTime + "ms");
+
+        // run the same method with different thread numbers
+        for (int threadCount : new int[] {2, 3, 4}) {
+            long time = memMulti(threadCount);
+            double timeFactor = 1.0 * singleTime * threadCount / time;
+            System.out.println(threadCount + " threads: " + time + "ms (" + timeFactor + "x)");
+        }
+    }
+
+    /**
+     * Creates and accesses a ARRAY_SIZE big byte[].
+     * @return time to create and access array in milliseconds
+     */
+    private static long mem() {
+        long start = System.currentTimeMillis();
+        final byte[] array = new byte[ARRAY_SIZE];
+        if (ACCESS_ARRAY) {
+            for (int i = 0; i < array.length; i++) {
+                //array[i] = (byte) 170; //write
+                byte x = array[i]; //read
+            }
+        }
+        return System.currentTimeMillis() - start;
+    }
+
+    /**
+     * Starts multiple threads and runs mem() in each one.
+     * @return total time for all threads
+     */
+    private static long memMulti(int numOfThreads) {
+        Thread[] threads = new Thread[numOfThreads];
+        long start = System.currentTimeMillis();
+
+        for (int i = 0; i < threads.length; i++) {
+            threads[i] = new Thread(new MemoryRamp(), "mem-");
+            threads[i].start();
+        }
+
+        try {
+            for (Thread thread : threads) {
+                thread.join();
+            }
+        }
+        catch (InterruptedException iex) {
+            throw new RuntimeException(iex);
+        }
+        return System.currentTimeMillis() - start;
+    }
+}