如何使用C#读写锁ReaderWriterLockSlim

2019-10-29 21:40:06

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这篇文章向大家介绍了读写锁ReaderWriterLockSlim，其优点就是多个线程可以同时读取该对象，要了解更多读写锁的知识，仔细阅读下文吧

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());

}

}

}

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：

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。