Operating Systems - Old Questions

1. Why is Optimal Page Replacement the best but not practically feasible page replacement algorithm? Calculate the number of page faults for Optimal, LRU, and FIFO replacement algorithms for the reference string: 1, 3, 4, 2, 3, 5, 4, 3, 1, 2, 4, 6, 3, 2, 1, 4, 2 using 3 page frames. [1+9]