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:

public class Program
    {
        static private ReaderWriterLockSlim rwl = new ReaderWriterLockSlim();
        static void Main(string[] args)
        {
            Thread t_read1 = new Thread(new ThreadStart(ReadSomething));
            t_read1.Start();
            Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read1.GetHashCode());
            Thread t_read2 = new Thread(new ThreadStart(ReadSomething));
            t_read2.Start();
            Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read2.GetHashCode());
            Thread t_write1 = new Thread(new ThreadStart(WriteSomething));
            t_write1.Start();
            Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write1.GetHashCode());
        }
        static public void ReadSomething()
        {
            Console.WriteLine("{0} Thread ID {1} Begin EnterReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            rwl.EnterReadLock();
            try
            {
                Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                Thread.Sleep(5000);//模拟读取信息
                Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            }
            finally
            {
                rwl.ExitReadLock();
                Console.WriteLine("{0} Thread ID {1} ExitReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            }
        }
        static public void WriteSomething()
        {
            Console.WriteLine("{0} Thread ID {1} Begin EnterWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            rwl.EnterWriteLock();
            try
            {
                Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                Thread.Sleep(10000);//模拟写入信息
                Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            }
            finally
            {
                rwl.ExitWriteLock();
                Console.WriteLine("{0} Thread ID {1} ExitWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            }
        }
    }

C#读写锁ReaderWriterLockSlim的使用_C#

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:

      

  static void Main(string[] args)
        {
            Thread t_write1 = new Thread(new ThreadStart(WriteSomething));
            t_write1.Start();
            Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write1.GetHashCode());
            Thread t_write2 = new Thread(new ThreadStart(WriteSomething));
            t_write2.Start();
            Console.WriteLine("{0} Create Thread ID {1} , Start WriteSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_write2.GetHashCode());
            Thread t_read1 = new Thread(new ThreadStart(ReadSomething));
            t_read1.Start();
            Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read1.GetHashCode());
            Thread t_read2 = new Thread(new ThreadStart(ReadSomething));
            t_read2.Start();
            Console.WriteLine("{0} Create Thread ID {1} , Start ReadSomething", DateTime.Now.ToString("hh:mm:ss fff"), t_read2.GetHashCode());
        }

结果如下:

C#读写锁ReaderWriterLockSlim的使用_读写锁_02

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。

static public void UpgradeableRead()
        {
            Console.WriteLine("{0} Thread ID {1} Begin EnterUpgradeableReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            rwl.EnterUpgradeableReadLock();
            try
            {
                Console.WriteLine("{0} Thread ID {1} doing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                Console.WriteLine("{0} Thread ID {1} Begin EnterWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                rwl.EnterWriteLock();
                try
                {
                    Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                    Thread.Sleep(10000);//模拟写入信息
                    Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                }
                finally
                {
                    rwl.ExitWriteLock();
                    Console.WriteLine("{0} Thread ID {1} ExitWriteLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                }
                Thread.Sleep(10000);//模拟读取信息
                Console.WriteLine("{0} Thread ID {1} doing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            }
            finally
            {
                rwl.ExitUpgradeableReadLock();
                Console.WriteLine("{0} Thread ID {1} ExitUpgradeableReadLock...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            }
        }


读写锁对于性能的影响是明显的。

下面测试代码:

public class Program
    {
        static private ReaderWriterLockSlim rwl = new ReaderWriterLockSlim();
        static void Main(string[] args)
        {
            Stopwatch sw = new Stopwatch();
            sw.Start();
            List<Task> lstTask = new List<Task>();
            for (int i = 0; i < 500; i++)
            {
                if (i % 25 != 0)
                {
                    var t = Task.Factory.StartNew(ReadSomething);
                    lstTask.Add(t);
                }
                else
                {
                    var t = 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 = new List<Task>();
            for (int i = 0; i < 500; i++)
            {
                if (i % 25 != 0)
                {
                    var t = Task.Factory.StartNew(ReadSomething_lock);
                    lstTask.Add(t);
                }
                else
                {
                    var t = 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);
        }
        static private object _lock1 = new object();
        static public void ReadSomething_lock()
        {
            lock (_lock1)
            {
                //Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                Thread.Sleep(10);//模拟读取信息
                //Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            }
        }
        static public void WriteSomething_lock()
        {
            lock (_lock1)
            {
                //Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                Thread.Sleep(100);//模拟写入信息
                //Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            }
        }
        static public void ReadSomething()
        {
            rwl.EnterReadLock();
            try
            {
                //Console.WriteLine("{0} Thread ID {1} reading sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                Thread.Sleep(10);//模拟读取信息
                //Console.WriteLine("{0} Thread ID {1} reading end.", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
            }
            finally
            {
                rwl.ExitReadLock();
            }
        }
        static public void WriteSomething()
        {
            rwl.EnterWriteLock();
            try
            {
                //Console.WriteLine("{0} Thread ID {1} writing sth...", DateTime.Now.ToString("hh:mm:ss fff"), Thread.CurrentThread.GetHashCode());
                Thread.Sleep(100);//模拟写入信息
                //Console.WriteLine("{0} Thread ID {1} writing end.", DateTime.Now.ToString("hh:mm:ss fff"), 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。

C#读写锁ReaderWriterLockSlim的使用_并发_03