Lecture 2

Question 1

How does the clock replacement algorithm try to implement LRU?

Answer 1

Try to achieve some of the benefits of LRU replacement, but without the computational overhead of LRU.

Question 2

How does the clock replacement algorithm approximate LRU?

Answer 2

Uses a reference bit. This reference bit corresponds to a used bit that is set for each page reference.

Question 3

What does it mean if the used bit is not set?

Answer 3

The corresponding page has not been referenced in a long time.

Question 4

Where is the used bit for each page usually stored?

Answer 4

Page table or Translation Look-aside Buffer(T LB)

Question 5

What makes the used bit fast to access and modify?

Answer 5

It is just one bit, stored in cache memory

Question 6

What are the three states the clock replacement algorithm assumes a page can be in when a page reference occurs?

Answer 6

1. Page not in memory
2. Used-bit = 0, Page in memory but not recently used.
3. Used bit = 1, Page in memory and recently used.

Question 7

Is it possible to have a page fault when used bit set to 1?

Answer 7

No, no page fault will occur when the page is being accessed.

Question 8

What happens when page fault occurs when used bit = 0?

Answer 8

Bit is changed to 1 (recently used).

Question 9

How can a page fault occur when used bit = 0?

Answer 9

The page is not in main memory but being temporarily stored in the translation look aside buffer (TLB).

Question 10

Which pages does the clock replacement algorithm end up replacing?

Answer 10

The clock algorithm thus replaces pages that have not been referenced for one complete revolution of the page pointer.

Question 11

How are the physical page frames in the clock algorithm arranged in a circle?

Answer 11

With a circular linked list.

Question 12

What is the goal of the clock algorithm?

Answer 12

Replace pages that are old enough and not necessarily the oldest

Question 13

How does the clock algorithm store pages?

Answer 13

In circular linked list, with one reference bit per page.

Question 14

What happens when the clock algorithm encounters a page with the reference bit set?

Answer 14

Set its reference bit to 0,and advance the clock pointer to the next page.

Question 15

Best case and worst case of clock replacement algorithm.

Answer 15

Best - First page has reference bit cleared and is replaced.
Worst - All pages have reference bit set, clock has to loop around entire list and replace the page where the pointer started.

Question 16

Advantage of the Clock replacement algorithm?

Answer 16

Maintains only one reference bit for each page, so it is fast and efficient.

Question 17

Disadvantage of Clock replacement algorithm?

Answer 17

reference bit only indicates if page was used at all since last checked and not how long it has been since last used.

Question 18

How to get around disadvantage of the Clock replacement algorithm?

Answer 18

Have extra reference bits associated with each page.
At each page reference, increment a 8−bit reference for each page to reflect how old a page is, where a page fault, the oldest page is replaced

Question 19

Disadvantage with 8-bit reference strategy?

Answer 19

A page fault will require a search through all the pages in the linked list,in order to be sure that the oldest page is replaced.

Question 20

Clock-hand moving slowly.

Answer 20

pointer passes few pages before a page is replaced. Many page faults, fewer reference bits set.

Question 21

Clock-hand moving fast.

Answer 21

Pointer passes many pages before a page is replaced.
Few page faults.
Reference bits set.

Question 22

Dirty Page

Answer 22

A page which is modified in a buffer cache (between swap-out from secondary storage and swap-in to a main memory frame).
Corresponding Dirty bit set in page table.

Question 23

What happens when dirty page is replaced?

Answer 23

Must be written to disk (e.g. hard-drive secondary storage) before its page frame is reused.