MaxDegreeOfParallelism做什么？

MaxDegreeOfParallelism做什么？

答案是，这是整个并行 *** 作的上限，与内核数无关。

因此，即使由于等待IO或锁定而没有使用CPU，也不会并行运行额外的任务，只会执行您指定的最大值。

为了找到答案，我编写了这段测试代码。那里有一个人为锁，以刺激TPL使用更多线程。当您的代码等待IO或数据库时，也会发生同样的情况。

class Program{    static void Main(string[] args)    {        var locker = new Object();        int count = 0;        Parallel.For (0  , 1000  , new ParallelOptions { MaxDegreeOfParallelism = 2 }  , (i) =>        { Interlocked.Increment(ref count); lock (locker) {     Console.WriteLine("Number of active threads:" + count);     Thread.Sleep(10);  }  Interlocked.Decrement(ref count);         } );    }}

如果我未指定MaxDegreeOfParallelism，则控制台日志将显示最多同时运行约8个任务。像这样：

Number of active threads:6Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:6Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7Number of active threads:7

它开始时较低，随时间增加，最后尝试同时运行8。

如果我将其限制为任意值（例如2），我得到

Number of active threads:2Number of active threads:1Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2Number of active threads:2

哦，这是在四核计算机上。