对不定行大小的大文件进行磁盘外排序，分片时用的是双缓冲和双线程，归并排序时使用单线程这一点可以作多线程优化，提高归并效率。-Large size of the variable line outside the sorting disk files, sub-films using a double-buffering and double thread, the use of single-threaded merge sort can be long on this thread optim