ReentrantLock Fair 与 Unfair 的巨大差异

ReentrantLock  可重入的锁是我们平常除了intrinsic  lock  （也就是 synchronized 方法， synchronized block）之外用得最多的了同步方式了。 一般情况下 我们用 ReentrantLock  的时候就是用它的默认建构函数方式 

     new ReentrantLock  ();

但其实它带一个 参数 是否 fair。如果是true  也就是FairSync 所在有多个线程同时竞争这个锁得时候， 会考虑公平性尽可能的让不同的线程公平。 这个公平其实是有很大的性能损失换来的。下面有个例子 :  \

 

package com.bwang.concurrent;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.CyclicBarrier;

import static java.lang.System.out;

public final class TestLocks implements Runnable
{
    public enum LockType { JVM, JUC }
    public static LockType lockType;

    public static final long ITERATIONS = 5L * 1000L * 1000L;
    public static long counter = 0L;

    public static final Object jvmLock = new Object();
    public static final Lock jucLock = new ReentrantLock(false);
    private static int numThreads;

    private final long iterationLimit;
    private final CyclicBarrier barrier;
    private long localCounter = 0L;
    public long getLocalCounter() 
    {
    	return localCounter;
    } 

    public TestLocks(final CyclicBarrier barrier, final long iterationLimit)
    {
        this.barrier = barrier;
        this.iterationLimit = iterationLimit;
    }

    public static void main(final String[] args) throws Exception
    {
        lockType = LockType.valueOf("JUC");
        numThreads = Integer.parseInt("8");

        final long start = System.nanoTime();
        runTest(numThreads, ITERATIONS);
        final long duration = System.nanoTime() - start;

        out.printf("%d threads, duration %,d (ns)\n", numThreads, duration);
        out.printf("%,d ns/op\n", duration / ITERATIONS);
        out.printf("%,d ops/s\n", (ITERATIONS * 1000000000L) / duration);
        out.println("counter = " + counter);
    }

    private static void runTest(final int numThreads, final long iterationLimit)
        throws Exception
    {
        CyclicBarrier barrier = new CyclicBarrier(numThreads);
        Thread[] threads = new Thread[numThreads];
        TestLocks[] testLocks = new TestLocks[numThreads];
        for (int i = 0; i < threads.length; i++)
        {
        	testLocks[i] = new TestLocks(barrier, iterationLimit);
            threads[i] = new Thread(testLocks[i]);
        }

        for (Thread t : threads)
        {
            t.start();
        }

        for (Thread t : threads)
        {
            t.join();
        }
        for (int i = 0; i < threads.length; i++)
        {
            out.printf("%d thread, local counter = %,d\n", i, testLocks[i].getLocalCounter());
        }
    }

    public void run()
    {
        try
        {
            barrier.await();
        }
        catch (Exception e)
        {
            // don't care
        }

        switch (lockType)
        {
            case JVM: jvmLockInc(); break;
            case JUC: jucLockInc(); break;
        }
    }

    private void jvmLockInc()
    {
        
        while (true)
        {
        	long count = 0;
            synchronized (jvmLock)
            {
                ++counter;
                count = counter;
            }
            localCounter++;
            if (count >= iterationLimit)  {
            	break;
            }
        }
    }

    private void jucLockInc()
    {
        while (true)
        {
            long count = 0L;
            jucLock.lock();
            try
            {
                ++counter;
                count = counter;
            }
            finally
            {
                jucLock.unlock();
            }
            localCounter++;
            if (count >= iterationLimit)  {
            	break;
            }
        }
    }
}

 

 

我们简单用N个线程来同步一个counter  5，000,000次。 如果是 new ReentrantLock(true) 也就是 FairSync 方式 ：

 

 

0 thread, local counter = 624,822

1 thread, local counter = 625,135

2 thread, local counter = 624,936

3 thread, local counter = 624,800

4 thread, local counter = 625,007

5 thread, local counter = 624,921

6 thread, local counter = 625,298

7 thread, local counter = 625,088

8 threads, duration 16,553,236,994 (ns)

3,310 ns/op

302,055 ops/s

counter = 5000007

 

 

可以看到8 个线程 每个线程的获取lock都很接近 但是它要 3310 个ns 来进行一次。   如果采用  如果是 new ReentrantLock(false) 就是 UnfairSync 方式：

0 thread, local counter = 626,786

1 thread, local counter = 594,983

2 thread, local counter = 590,274

3 thread, local counter = 688,725

4 thread, local counter = 588,090

5 thread, local counter = 586,885

6 thread, local counter = 732,210

7 thread, local counter = 592,054

8 threads, duration 425,844,254 (ns)

85 ns/op

11,741,381 ops/s

counter = 5000007

虽然 每个thread 获取lock 的次数差异很大 从   592，054到  732,210，  但每次操作自需要 85 ns。  3310 对 85 这个差异太大聊。  

如果我们用intrinsic lock 的方法 结果如下：

 

0 thread, local counter = 498,363

1 thread, local counter = 512,603

2 thread, local counter = 799,367

3 thread, local counter = 500,946

4 thread, local counter = 824,935

5 thread, local counter = 652,921

6 thread, local counter = 692,219

7 thread, local counter = 518,653

8 threads, duration 877,777,848 (ns)

175 ns/op

5,696,202 ops/s

counter = 5000007

 

intrinsic lock 也应该是unfair 的方式， 每个线程获取的机会差异比较大， 每个操作需要 175ns。  比 unfair 的 ReentrantLock  性能差些。   

 

得出的结果是 如果我们仅考虑同步锁得性能不需要考虑公平性优先考虑 

      new ReentrantLock(false)    

再次是 intrinsic lock

万不得已的必须要FairSync 的情况下才用 new ReentrantLock(true)。