Question

1- Stack algorithms are a class of page replacement algorithms that

Select one:

a. are guaranteed to incur no more page faults than FIFO page replacement algorithm.

b. do not suffer from Belady’s anomaly.

c. are implemented using stacks.

d. are guaranteed to incur the least number of page faults.

2- Which of the following is true about the strategy that uses page fault frequency (PFF) to prevent thrashing?

Select one:

a. A new page is allocated to a process if PFF is too high.

b. A page is deallocated from a process if the PFF is too low.

c. All of the above.

d. A new process may be swapped in if PFF is too low.

Answer 1

1. Option b) is correct.

Stack based algorithms do not suffer from Belady's Anomaly(number of page faults may increase when we increase the number of frames). This is because for an example consider, LRU page replacement algorithm, in which the most recently used N pages will be there in the memory and even if we increase the number of frames, these N pages will still be there in the memory, so it does not suffer from the Belady's Anomaly.

2. High page fault implies, that very few frames are allocated to the process and if PFF is too low, this implies process has too many frames allocated to it. To prevent thrashing, we must control the PFF. Our aim is to maintain the PFF, therefore, according to these definitions we conclude that all of the above are correct.

Correct choice is option c)