Use multi-threads can speed up this program

[iamsurfing]

Files which have same size can create a thread.
Without multi-threads, it is too slow.

[lededev]

If files are on HDD, multi-threading will only slow down the speed because it will keep the head moving back and forth, and the time will be spent on the head seek; If files are on SSD, and most of them are small, it is possible to increase the processing speed.