读写锁的概念很简单,允许多个线程同时获取读锁,但同一时间只允许一个线程获得写锁,因此也称作共享-独占锁。在C#中,推荐使用ReaderWriterLockSlim类来完成读写锁的功能。

某些场合下,对一个对象的读取次数远远大于修改次数,如果只是简单的用lock方式加锁,则会影响读取的效率。而如果采用读写锁,则多个线程可以同时读取该对象,只有等到对象被写入锁占用的时候,才会阻塞。

简单的说,当某个线程进入读取模式时,此时其他线程依然能进入读取模式,假设此时一个线程要进入写入模式,那么他不得不被阻塞。直到读取模式退出为止。

同样的,如果某个线程进入了写入模式,那么其他线程无论是要写入还是读取,都是会被阻塞的。

进入写入/读取模式有2种方法:

EnterReadLock尝试进入写入模式锁定状态。

TryEnterReadLock(Int32) 尝试进入读取模式锁定状态,可以选择整数超时时间。

EnterWriteLock 尝试进入写入模式锁定状态。

TryEnterWriteLock(Int32) 尝试进入写入模式锁定状态,可以选择超时时间。

退出写入/读取模式有2种方法:

ExitReadLock 减少读取模式的递归计数,并在生成的计数为 0(零)时退出读取模式。

ExitWriteLock 减少写入模式的递归计数,并在生成的计数为 0(零)时退出写入模式。

下面演示一下用法:

publicclassProgram{staticprivateReaderWriterLockSlimrwl=newReaderWriterLockSlim();staticvoidMain(string[]args){Threadt_read1=newThread(newThreadStart(ReadSomething));t_read1.Start();Console.WriteLine("{0}CreateThreadID{1},StartReadSomething",DateTime.Now.ToString("hh:mm:ssfff"),t_read1.GetHashCode());Threadt_read2=newThread(newThreadStart(ReadSomething));t_read2.Start();Console.WriteLine("{0}CreateThreadID{1},StartReadSomething",DateTime.Now.ToString("hh:mm:ssfff"),t_read2.GetHashCode());Threadt_write1=newThread(newThreadStart(WriteSomething));t_write1.Start();Console.WriteLine("{0}CreateThreadID{1},StartWriteSomething",DateTime.Now.ToString("hh:mm:ssfff"),t_write1.GetHashCode());}staticpublicvoidReadSomething(){Console.WriteLine("{0}ThreadID{1}BeginEnterReadLock...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());rwl.EnterReadLock();try{Console.WriteLine("{0}ThreadID{1}readingsth...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());Thread.Sleep(5000);//模拟读取信息Console.WriteLine("{0}ThreadID{1}readingend.",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}finally{rwl.ExitReadLock();Console.WriteLine("{0}ThreadID{1}ExitReadLock...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}}staticpublicvoidWriteSomething(){Console.WriteLine("{0}ThreadID{1}BeginEnterWriteLock...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());rwl.EnterWriteLock();try{Console.WriteLine("{0}ThreadID{1}writingsth...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());Thread.Sleep(10000);//模拟写入信息Console.WriteLine("{0}ThreadID{1}writingend.",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}finally{rwl.ExitWriteLock();Console.WriteLine("{0}ThreadID{1}ExitWriteLock...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}}}

可以看到3号线程和4号线程能够同时进入读模式,而5号线程过了5秒钟后(即3,4号线程退出读锁后),才能进入写模式。

把上述代码修改一下,先开启2个写模式的线程,然后在开启读模式线程,代码如下:

staticvoidMain(string[]args){Threadt_write1=newThread(newThreadStart(WriteSomething));t_write1.Start();Console.WriteLine("{0}CreateThreadID{1},StartWriteSomething",DateTime.Now.ToString("hh:mm:ssfff"),t_write1.GetHashCode());Threadt_write2=newThread(newThreadStart(WriteSomething));t_write2.Start();Console.WriteLine("{0}CreateThreadID{1},StartWriteSomething",DateTime.Now.ToString("hh:mm:ssfff"),t_write2.GetHashCode());Threadt_read1=newThread(newThreadStart(ReadSomething));t_read1.Start();Console.WriteLine("{0}CreateThreadID{1},StartReadSomething",DateTime.Now.ToString("hh:mm:ssfff"),t_read1.GetHashCode());Threadt_read2=newThread(newThreadStart(ReadSomething));t_read2.Start();Console.WriteLine("{0}CreateThreadID{1},StartReadSomething",DateTime.Now.ToString("hh:mm:ssfff"),t_read2.GetHashCode());}

结果如下:

可以看到,3号线程和4号线程都要进入写模式,但是3号线程先占用写入锁,因此4号线程不得不等了10s后才进入。5号线程和6号线程需要占用读取锁,因此等4号线程退出写入锁后才能继续下去。

TryEnterReadLock和TryEnterWriteLock可以设置一个超时时间,运行到这句话的时候,线程会阻塞在此,如果此时能占用锁,那么返回true,如果到超时时间还未占用锁,那么返回false,放弃锁的占用,直接继续执行下面的代码。

EnterUpgradeableReadLock

ReaderWriterLockSlim类提供了可升级读模式,这种方式和读模式的区别在于它还有通过调用 EnterWriteLock 或 TryEnterWriteLock 方法升级为写入模式。 因为每次只能有一个线程处于可升级模式。进入可升级模式的线程,不会影响读取模式的线程,即当一个线程进入可升级模式,任意数量线程可以同时进入读取模式,不会阻塞。如果有多个线程已经在等待获取写入锁,那么运行EnterUpgradeableReadLock将会阻塞,直到那些线程超时或者退出写入锁。

下面代码演示了如何在可升级读模式下,升级到写入锁。

staticpublicvoidUpgradeableRead(){Console.WriteLine("{0}ThreadID{1}BeginEnterUpgradeableReadLock...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());rwl.EnterUpgradeableReadLock();try{Console.WriteLine("{0}ThreadID{1}doingsth...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());Console.WriteLine("{0}ThreadID{1}BeginEnterWriteLock...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());rwl.EnterWriteLock();try{Console.WriteLine("{0}ThreadID{1}writingsth...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());Thread.Sleep(10000);//模拟写入信息Console.WriteLine("{0}ThreadID{1}writingend.",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}finally{rwl.ExitWriteLock();Console.WriteLine("{0}ThreadID{1}ExitWriteLock...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}Thread.Sleep(10000);//模拟读取信息Console.WriteLine("{0}ThreadID{1}doingend.",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}finally{rwl.ExitUpgradeableReadLock();Console.WriteLine("{0}ThreadID{1}ExitUpgradeableReadLock...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}}


读写锁对于性能的影响是明显的。

下面测试代码:

publicclassProgram{staticprivateReaderWriterLockSlimrwl=newReaderWriterLockSlim();staticvoidMain(string[]args){Stopwatchsw=newStopwatch();sw.Start();List<Task>lstTask=newList<Task>();for(inti=0;i<500;i++){if(i%25!=0){vart=Task.Factory.StartNew(ReadSomething);lstTask.Add(t);}else{vart=Task.Factory.StartNew(WriteSomething);lstTask.Add(t);}}Task.WaitAll(lstTask.ToArray());sw.Stop();Console.WriteLine("使用ReaderWriterLockSlim方式,耗时:"+sw.Elapsed);sw.Restart();lstTask=newList<Task>();for(inti=0;i<500;i++){if(i%25!=0){vart=Task.Factory.StartNew(ReadSomething_lock);lstTask.Add(t);}else{vart=Task.Factory.StartNew(WriteSomething_lock);lstTask.Add(t);}}Task.WaitAll(lstTask.ToArray());sw.Stop();Console.WriteLine("使用lock方式,耗时:"+sw.Elapsed);}staticprivateobject_lock1=newobject();staticpublicvoidReadSomething_lock(){lock(_lock1){//Console.WriteLine("{0}ThreadID{1}readingsth...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());Thread.Sleep(10);//模拟读取信息//Console.WriteLine("{0}ThreadID{1}readingend.",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}}staticpublicvoidWriteSomething_lock(){lock(_lock1){//Console.WriteLine("{0}ThreadID{1}writingsth...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());Thread.Sleep(100);//模拟写入信息//Console.WriteLine("{0}ThreadID{1}writingend.",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}}staticpublicvoidReadSomething(){rwl.EnterReadLock();try{//Console.WriteLine("{0}ThreadID{1}readingsth...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());Thread.Sleep(10);//模拟读取信息//Console.WriteLine("{0}ThreadID{1}readingend.",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}finally{rwl.ExitReadLock();}}staticpublicvoidWriteSomething(){rwl.EnterWriteLock();try{//Console.WriteLine("{0}ThreadID{1}writingsth...",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());Thread.Sleep(100);//模拟写入信息//Console.WriteLine("{0}ThreadID{1}writingend.",DateTime.Now.ToString("hh:mm:ssfff"),Thread.CurrentThread.GetHashCode());}finally{rwl.ExitWriteLock();}}}

上述代码,就500个Task,每个Task占用一个线程池线程,其中20个写入线程和480个读取线程,模拟操作。其中读取数据花10ms,写入操作花100ms,分别测试了对于lock方式和ReaderWriterLockSlim方式。可以做一个估算,对于ReaderWriterLockSlim,假设480个线程同时读取,那么消耗10ms,20个写入操作占用2000ms,因此所消耗时间2010ms,而对于普通的lock方式,由于都是独占性的,因此480个读取操作占时间4800ms+20个写入操作2000ms=6800ms。运行结果显示了性能提升明显。