饥饿线程

最新推荐文章于 2025-05-21 10:08:37 发布

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#StampedLock #饥饿线程

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本文探讨了线程饥饿现象，即低优先级线程因高优先级线程持续占用CPU资源而无法运行的问题。通过Java代码示例，对比了ReentrantReadWriteLock与StampedLock在解决线程饥饿上的表现，介绍了StampedLock的乐观锁机制及其局限性。

定义

如果一个线程因为CPU被其他线程全部占有而始终抢不到CPU的运行时间，那么这个线程就是饥饿线程。

实例

由于线程t1的优先级为9，线程t2的优先级为1（最低），造成t2很难抢到CPU的运行时间。


import java.util.concurrent.TimeUnit;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class HungryThread{
    ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
    void write(){
        readWriteLock.writeLock().lock();
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        readWriteLock.writeLock().unlock();
    }
    void read(){
        readWriteLock.readLock().lock();
        System.out.println("read");
        readWriteLock.readLock().unlock();
    }

    public static void main(String[] args) {
        HungryThread hungryThread = new HungryThread();

        Thread t1 = new Thread(()->{
            //不停去拿写锁，拿到后sleep一段时间，释放
            while (true) {
                hungryThread.write();
            }
        });

        Thread t2 = new Thread(()->{
            //不停去拿读锁，虽然是读锁，但是...看下面！
            while (true){
                hungryThread.read();
            }
        });
		
        t1.setPriority(9);
        //优先级低！
        t2.setPriority(1);

        t1.start();
        t2.start();
    }
}

解决方案 StampedLock

采用乐观锁机制，判断有没有线程占据锁，没有的话直接执行。


import java.util.concurrent.locks.StampedLock;

public class StampedThread {
    StampedLock lock = new StampedLock();
    void write(){
        long stamp = lock.writeLock();
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        lock.unlock(stamp);
    }
    void read(){
        //乐观读
        long stamp = lock.tryOptimisticRead();
        //判断是否有写在进行，没占用的话，得到执行，打印read
        if (lock.validate(stamp)){
            System.out.println("read");
        }
    }

    public static void main(String[] args) {
        StampedThread stampedThread = new StampedThread();

        Thread t1 = new Thread(()->{
            while (true) {
                stampedThread.write();
            }
        });

        Thread t2 = new Thread(()->{
            while (true){
                stampedThread.read();
            }
        });

        t1.setPriority(9);
        t2.setPriority(1);

        t1.start();
        t2.start();
    }
}