c# 并发实际运行线程数目小于预期数目

写了一段代码预期应该是一秒钟内输出10个时间,实际上输出10个时间大概在3秒钟左右,代码如下
ParallelOptions opt = new ParallelOptions()
{
MaxDegreeOfParallelism = 10
};
Parallel.For(0, 10, opt, i =>
{
Console.WriteLine(DateTime.Now.TimeOfDay.ToString());
Thread.Sleep(5000);

        });

MaxDegreeOfParallelism值设定超过5,好像就没有作用了,新手求指点

http://www.jb51.net/article/89516.htm