Java Biased Lock 实验

今天神舟七号升天，不知道是不是去成都的飞机都受影响，19：50的飞机改到23：20，在机场无聊，写篇博客吧

什么是biased lock?
Biased Locking is a class of optimizations that improves uncontended synchronization performance by eliminating atomic operations associated with the Java language’s synchronization primitives.

有些时候一个线程多次获得对象锁的操作中，理论上这些操作可以合并在一起，而减少lock/unlock的时间。不过这样会导致对其他线程的不公平，所以叫biased lock.通常是对第一个获得锁的线程偏心。

禁止：-XX:-UseBiasedLocking
使用：JRE6默认使用，或者显式使用-XX:+UseBiasedLocking
以下是我的测试程序，在JRE6上使用biased lock只需要500毫秒左右，而使用JRE5则需要5500毫秒左右，快10倍以上。
奇怪的是JRE6上禁止biased lock之后貌似性能没有什么变化,如果你打印一些调试信息（关闭注释System.out.println("T" + id + " holds lock")那行）你会发现仍然是biased lock，知道一个线程结束才释放锁。

package test.thread;

import java.util.concurrent.CountDownLatch;

public class BiasedLockTest {

public static volatile Long t1 = System.nanoTime();
static {
System.out.println(t1);
}
public static volatile Long t2;

public static void main(String[] args) throws InterruptedException {

Object lock = new Object();

TestThread[] threads = new TestThread[100];
CountDownLatch latch = new CountDownLatch(threads.length);

for (int i = 0; i < 100; i++) {
threads[i] = new TestThread(i, lock, latch);
}
for (int i = 0; i < 100; i++) {
threads[i].start();
}

for (int i = 0; i < 100; i++) {
threads[i].join();
}
System.out.println(t2);
System.out.println( (t2 - t1) / 10e5);
}
}


class TestThread extends Thread {
final private int id;
final private Object lock;
final private CountDownLatch latch;

public TestThread(int id, Object lock, CountDownLatch latch) {
this.id = id;
this.lock = lock;
this.latch = latch;
}
public void run() {
for(int i = 0; i < 10000; i++) {
synchronized (lock) {
int k = 0;
k=k+1;//dummy calculation
}
// System.out.println("T" + id + " holds lock");
}
BiasedLockTest.t2 = System.nanoTime();
latch.countDown();
// System.out.println("T" + id + " dies");
}
}