作者：wenyinfeng
转载时，请注明原文出处，谢谢！
//: concurrency/SynchronizationComparisons.java
// Comparing the performance of explicit Locks
// and Atomics versus the synchronized keyword.
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.concurrent.locks.*;
import java.util.*;
import static net.mindview.util.Print.*;
abstract class Accumulator {
public static long cycles = 50000L;
// Number of Modifiers and Readers during each test:
private static final int N = 4;
public static ExecutorService exec =
Executors.newFixedThreadPool(N*2);
private static CyclicBarrier barrier =
new CyclicBarrier(N*2 + 1);
protected volatile int index = 0;
protected volatile long value = 0;
protected long duration = 0;
protected String id = "error";
protected final static int SIZE = 100000;
protected static int[] preLoaded = new int[SIZE];
static {
// Load the array of random numbers:
Random rand = new Random(47);
for(int i = 0; i < SIZE; i++)
preLoaded[i] = rand.nextInt();
}
public abstract void accumulate();
public abstract long read();
private class Modifier implements Runnable {
public void run() {
for(long i = 0; i < cycles; i++)
accumulate();
try {
barrier.await();
} catch(Exception e) {
throw new RuntimeException(e);
}
}
}
private class Reader implements Runnable {
private volatile long value;
public void run() {
for(long i = 0; i < cycles; i++)
value = read();
try {
barrier.await();
} catch(Exception e) {
throw new RuntimeException(e);
}
}
}
public void timedTest() {
long start = System.nanoTime();
for(int i = 0; i < N; i++) {
exec.execute(new Modifier());
exec.execute(new Reader());
}
try {
barrier.await();
} catch(Exception e) {
throw new RuntimeException(e);
}
duration = System.nanoTime() - start;
printf("%-13s: d\n", id, duration);
}
public static void
report(Accumulator acc1, Accumulator acc2) {
printf("%-22s: %.2f\n", acc1.id + "/" + acc2.id,
(double)acc1.duration/(double)acc2.duration);
}
}
class BaseLine extends Accumulator {
{ id = "BaseLine"; }
public void accumulate() {
value += preLoaded[index++];
if(index >= SIZE) index = 0;
}
public long read() { return value; }
}
class SynchronizedTest extends Accumulator {
{ id = "synchronized"; }
public synchronized void accumulate() {
value += preLoaded[index++];
if(index >= SIZE) index = 0;
}
public synchronized long read() {
return value;
}
}
class LockTest extends Accumulator {
{ id = "Lock"; }
private Lock lock = new ReentrantLock();
public void accumulate() {
lock.lock();
try {
value += preLoaded[index++];
if(index >= SIZE) index = 0;
} finally {
lock.unlock();
}
}
public long read() {
lock.lock();
try {
return value;
} finally {
lock.unlock();
}
}
}
class AtomicTest extends Accumulator {
{ id = "Atomic"; }
private AtomicInteger index = new AtomicInteger(0);
private AtomicLong value = new AtomicLong(0);
public void accumulate() {
// Oops! Relying on more than one Atomic at
// a time doesn't work. But it still gives us
// a performance indicator:
int i = index.getAndIncrement();
value.getAndAdd(preLoaded[i]);
if(++i >= SIZE)
index.set(0);
}
public long read() { return value.get(); }
}
public class SynchronizationComparisons {
static BaseLine baseLine = new BaseLine();
static SynchronizedTest synch = new SynchronizedTest();
static LockTest lock = new LockTest();
static AtomicTest atomic = new AtomicTest();
static void test() {
print("============================");
printf("%-12s : d\n", "Cycles", Accumulator.cycles);
baseLine.timedTest();
synch.timedTest();
lock.timedTest();
atomic.timedTest();
Accumulator.report(synch, baseLine);
Accumulator.report(lock, baseLine);
Accumulator.report(atomic, baseLine);
Accumulator.report(synch, lock);
Accumulator.report(synch, atomic);
Accumulator.report(lock, atomic);
}
public static void main(String[] args) {
int iterations = 5; // Default
if(args.length > 0) // Optionally change iterations
iterations = new Integer(args[0]);
// The first time fills the thread pool:
print("Warmup");
baseLine.timedTest();
// Now the initial test doesn't include the cost
// of starting the threads for the first time.
// Produce multiple data points:
for(int i = 0; i < iterations; i++) {
test();
Accumulator.cycles *= 2;
}
Accumulator.exec.shutdown();
}
} /* Output: (Sample)
上面粗体的地方，你看出有什么问题了吗？
由于 BaseLine 和 AtomicTest 并没有对accumulate() 方法同步，而作者写的代码没有考虑到多线程修改 index 的问题，可能导致index 越界 ，程序崩溃！ 在这个示例代码中一次 timedTest() 测试会启动N个 Modifier 对象(在N个线程中)同时对同一个 index 进行修改，可能存在某个线程对 index++ 完成后任务被中断(判断 index 是否越界和置0的代码还没有被执行) ，另外一个线程又调用了 index++，这样index 就有可能超出SIZE的大小！
在我的机子上(jre6 + Intel i5 两核处理器)，运行几次都会出现越界崩溃！
由于这是性能测试，所以不能加锁，但可以通过赋值到临时变量i，并提前进行越界判断调整i和index (见下)，使得程序能够避免崩溃，正常进行性能测试。
public void accumulate() {
int i = index++;
if(i >= SIZE){
index = 0;
i = 0;
}
value += preLoaded[i];
}
public void accumulate() {
int i = index.getAndIncrement();
if(i >= SIZE){
i = 0;
index.set(0);
}
value.getAndAdd(preLoaded[i]);
}