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)。