慎用.NET2.0 ThreadPool

在一般观念中，ThreadPool管理了一些预先初始化好的线程，ThreadPool接受任务后，多数情况下能省去了线程创建的开销，更快地开始执行。但.NET 2.0 ThreadPool的表现也许和您的期望有所差距。下面的程序专门测试ThreadPool从接受任务到开始执行的时间延迟：

 测试1：并发创建10个sleep 500ms的任务，对比ThreadPool启动时间延迟和创建新线程的时间延迟。注：ThreadPool最小空闲线程数设为10。

 1 static void Main(string[] args)
 2 {
 3     ThreadPool.SetMinThreads(10, 1000);
 4 
 5     List<Stopwatch> watches = new List<Stopwatch>();
 6 
 7     for (int i = 0; i < 10; i++)
 8     {
 9         watches.Add(new Stopwatch());
         watches[i].Start();
 
         ThreadPool.QueueUserWorkItem(delegate(object obj)
         {
             int id = (int)obj;
 
             watches[id].Stop();
 
             Console.WriteLine("Pool Thread {0}, start delay: {1}ms", id, watches[id].ElapsedMilliseconds);
 
             Thread.Sleep(500);
         }, i);
     }
 
     Thread.Sleep(5 * 1000);
 
     Console.WriteLine("Press Key to Continue Test ");
 
     Console.ReadLine();
 
     watches.Clear();
     for (int i = 0; i < 10; i++)
     {
         watches.Add(new Stopwatch());
 
         watches[i].Start();
 
         Thread thread = new Thread(delegate(object obj)
         {
             int id = (int)obj;
 
             watches[id].Stop();
 
             Console.WriteLine("Create New Thread {0}, start delay: {1}ms", id, watches[id].ElapsedMilliseconds);
 
             Thread.Sleep(500);
         });
 
         thread.Start(i);
     }
 
 
     Console.ReadLine();
 }

 

结果1：

Pool Thread 0, start delay: 6ms
Pool Thread 2, start delay: 9ms
Pool Thread 1, start delay: 5ms
Pool Thread 3, start delay: 508ms
Pool Thread 4, start delay: 508ms
Pool Thread 5, start delay: 509ms
Pool Thread 6, start delay: 1005ms
Pool Thread 7, start delay: 1008ms
Pool Thread 8, start delay: 1008ms
Pool Thread 9, start delay: 1009ms

Press Key to Continue Test...

Create New Thread 0, start delay: 25ms

Create New Thread 1, start delay: 25ms
Create New Thread 2, start delay: 5ms
Create New Thread 3, start delay: 4ms
Create New Thread 4, start delay: 4ms
Create New Thread 5, start delay: 5ms
Create New Thread 6, start delay: 4ms
Create New Thread 7, start delay: 3ms
Create New Thread 8, start delay: 3ms
Create New Thread 9, start delay: 3ms

怎么样，是不是很让人惊讶，ThreadPool的任务启动延迟居然到了1000ms左右！？最少也是6ms，而新线程方式的任务启动延迟一般才5ms左右。另外，ThreadPool的SetMinThreads似乎不起作用，前面3个任务启动在几ms，我期望的是10个都应该启动很快。

测试2:  并发创建10个sleep 10ms的任务，对比ThreadPool启动时间延迟和创建新线程的时间延迟。注：ThreadPool最小空闲线程数设为10。

 1 static void Main(string[] args)
 2 {
 3     ThreadPool.SetMinThreads(10, 1000);
 4 
 5     List<Stopwatch> watches = new List<Stopwatch>();
 6 
 7     for (int i = 0; i < 10; i++)
 8     {
 9         watches.Add(new Stopwatch());
         watches[i].Start();
 
         ThreadPool.QueueUserWorkItem(delegate(object obj)
         {
             int id = (int)obj;
 
             watches[id].Stop();
 
             Console.WriteLine("Pool Thread {0}, start delay: {1}ms", id, watches[id].ElapsedMilliseconds);
 
             Thread.Sleep(10);
         }, i);
     }
 
     Thread.Sleep(5 * 1000);
 
     Console.WriteLine("Press Key to Continue Test ");
 
     Console.ReadLine();
 
     watches.Clear();
     for (int i = 0; i < 10; i++)
     {
         watches.Add(new Stopwatch());
 
         watches[i].Start();
 
         Thread thread = new Thread(delegate(object obj)
         {
             int id = (int)obj;
 
             watches[id].Stop();
 
             Console.WriteLine("Create New Thread {0}, start delay: {1}ms", id, watches[id].ElapsedMilliseconds);
 
             Thread.Sleep(10);
         });
 
         thread.Start(i);
     }
 
 
     Console.ReadLine();
 }

 

结果2：

Pool Thread 0, start delay: 1ms
Pool Thread 2, start delay: 29ms
Pool Thread 3, start delay: 29ms
Pool Thread 1, start delay: 28ms
Pool Thread 4, start delay: 39ms
Pool Thread 5, start delay: 39ms
Pool Thread 6, start delay: 40ms
Pool Thread 7, start delay: 40ms
Pool Thread 8, start delay: 49ms
Pool Thread 9, start delay: 49ms
Press Key to Continue Test...

Create New Thread 0, start delay: 9ms
Create New Thread 1, start delay: 10ms
Create New Thread 2, start delay: 11ms
Create New Thread 3, start delay: 3ms
Create New Thread 4, start delay: 3ms
Create New Thread 5, start delay: 3ms
Create New Thread 6, start delay: 3ms
Create New Thread 8, start delay: 46ms
Create New Thread 7, start delay: 97ms

Create New Thread 9, start delay: 90ms

这次ThreadPool延迟没有那么大，看来延迟和任务执行时间也有关系，如果ThreadPool中有长任务，会影响到后来任务的启动延迟。

测试3：依次创建10个sleep 500ms的任务，对比ThreadPool启动时间延迟和创建新线程的时间延迟。注：ThreadPool最小空闲线程数设为10。

 1 static void Main(string[] args)
 2 {
 3     ThreadPool.SetMinThreads(10, 1000);
 4 
 5     List<Stopwatch> watches = new List<Stopwatch>();
 6 
 7     for (int i = 0; i < 10; i++)
 8     {
 9         watches.Add(new Stopwatch());
         watches[i].Start();
 
         ThreadPool.QueueUserWorkItem(delegate(object obj)
         {
             int id = (int)obj;
 
             watches[id].Stop();
 
             Console.WriteLine("Pool Thread {0}, start delay: {1}ms", id, watches[id].ElapsedMilliseconds);
 
             Thread.Sleep(500);
         }, i);
 
         Thread.Sleep(500); //依次创建线程
     }
 
     Thread.Sleep(5 * 1000);
 
     Console.WriteLine("Press Key to Continue Test ");
 
     Console.ReadLine();
 
     watches.Clear();
     for (int i = 0; i < 10; i++)
     {
         watches.Add(new Stopwatch());
 
         watches[i].Start();
 
         Thread thread = new Thread(delegate(object obj)
         {
             int id = (int)obj;
 
             watches[id].Stop();
 
             Console.WriteLine("Create New Thread {0}, start delay: {1}ms", id, watches[id].ElapsedMilliseconds);
 
             Thread.Sleep(500);
         });
 
         thread.Start(i);
 
         Thread.Sleep(500); //依次创建线程
     }
 
 
     Console.ReadLine();
 }

 

结果2：

Pool Thread 0, start delay: 1ms
Pool Thread 1, start delay: 2ms
Pool Thread 2, start delay: 0ms
Pool Thread 3, start delay: 0ms
Pool Thread 4, start delay: 0ms
Pool Thread 5, start delay: 0ms
Pool Thread 6, start delay: 0ms
Pool Thread 7, start delay: 0ms
Pool Thread 8, start delay: 0ms
Pool Thread 9, start delay: 0ms
Press Key to Continue Test...

Create New Thread 0, start delay: 3ms
Create New Thread 1, start delay: 2ms
Create New Thread 2, start delay: 1ms
Create New Thread 3, start delay: 1ms
Create New Thread 4, start delay: 1ms
Create New Thread 5, start delay: 1ms
Create New Thread 6, start delay: 1ms
Create New Thread 7, start delay: 1ms
Create New Thread 8, start delay: 1ms

Create New Thread 9, start delay: 2ms

依次执行任务，这次ThreadPool表现良好，但也没有看出和创建新线程相比有多大优势。

结论：.NET2.0 ThreadPool的表现也许不符合大多数人对于ThreadPool的期望，慎用！

补充：后来从网上看到资料，确认是.NET2.0 ThreadPool的一个bug，请参考http://www.cnblogs.com/anhr/archive/2008/05/24/ThreadPool_BUG_in_DotNET_2_0_SP1.html，文章提供了一个work around，就是在调用ThreadPool.QueryWorkItem后马上调用Thread.Sleep(1)，经过我的测试有效果。