Consider a system with three frames in memory and following memory references in the working set

2 1 2 3 5 4 1 3 4 2 1

How many page fault will be there if we use second chance page replacement algorithm ?

The correct answer is Option 1: 7.

Key Points

• The Second Chance Page Replacement Algorithm is a modification of the FIFO (First In First Out) algorithm that reduces the number of page faults by giving pages a second chance to stay in memory if they have been referenced recently.
• When a page needs to be replaced, the algorithm inspects the reference bit of the pages in the order they were loaded into memory (FIFO order).
• If a page's reference bit is 0, it is replaced. If the reference bit is 1, the bit is cleared and the page is given a second chance, moving to the back of the queue.
• Consider the memory references in the working set: 2, 1, 2, 3, 5, 4, 1, 3, 4, 2, 1 with three frames in memory.
• The detailed steps are as follows:
  • Initially, all frames are empty.
  • Reference 2: Page 2 is loaded into the frame. (Page Fault)
  • Reference 1: Page 1 is loaded into the frame. (Page Fault)
  • Reference 2: Page 2 is already in the frame. (No Page Fault)
  • Reference 3: Page 3 is loaded into the frame. (Page Fault)
  • Reference 5: Page 2 is given a second chance (reference bit cleared), Page 1 is given a second chance (reference bit cleared), Page 3 is replaced by Page 5. (Page Fault)
  • Reference 4: Page 2 is given a second chance, Page 1 is given a second chance, Page 5 is replaced by Page 4. (Page Fault)
  • Reference 1: Page 1 is already in the frame. (No Page Fault)
  • Reference 3: Page 2 is given a second chance, Page 1 is given a second chance, Page 4 is replaced by Page 3. (Page Fault)
  • Reference 4: Page 2 is given a second chance, Page 1 is given a second chance, Page 3 is given a second chance (reference bit cleared), Page 4 is loaded into the frame. (Page Fault)
  • Reference 2: Page 2 is replaced by Page 2. (Page Fault)
  • Reference 1: Page 1 is already in the frame. (No Page Fault)
• In total, there are 7 page faults.

Additional Information

• The Second Chance Algorithm is also known as the Clock Algorithm because it can be visualized as a circular queue with a clock hand pointing to the next page to be considered for replacement.
• This algorithm helps reduce page faults by considering whether a page has been used recently before deciding to replace it.