参考网址: https://www.jb51.net/article/69869.htm
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:
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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());
}
}
}
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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:
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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());
}
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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。
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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());
}
}
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读写锁对于性能的影响是明显的。
下面测试代码:
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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();
}
}
}
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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。
以上是本文的全部内容,希望对大家熟练应用读写锁ReaderWriterLockSlim有所帮助。