多线程计算pi，并且做性能分析。单线程与多线程对比

计算量相同，线程数不同

例如，N取1000,000，测试使用1、2、3、4……个线程时所需要的时间。

线程数相同，计算量不同

例如，只考察单线程和双线程的性能对比，N分别取不同的数值。

-Multi-threaded computing pi, and do performance analysis. Comparison of single-threaded and multithreaded computing the same number of threads of different example, N take 1000,000, tested and 1,2,3,4 ... ... threads when required. The same number of threads, different calculation, for example, examine only single-threaded and dual-thread performance comparison, N respectively, for different values.