1. ICS 143 - Principles of Operating Systems
Discussion 10 - HW4 review
Joe Nash

2. Homework 4 Q1 Time 1 2 3 4 5 6 7 8 9 10 11 12 RS e c b a g d F0 F1 F2
Page fault? x -
1. Optimal
Total page fault: 7

3. Homework 4 Q1 Time 1 2 3 4 5 6 7 8 9 10 11 12 RS e c b a g d F0 F1 F2
Page fault? x -
2. LRU
Total page fault: 9

4. Homework 4 Q1 Time 1 2 3 4 5 6 7 8 9 10 11 12 RS e c b a g d F0 F1 F2
Page fault? x -
3. LRU approximation algorithm: Second chance
Total page fault: 8

5. Homework 4 Q2 Paging and TLB (24 points)
A computer keeps its page tables in memory. The overhead required for reading a value from memory is 3 microseconds. The computer has TLB, which holds pairs (virtual page, physical page frame).
What is the effective access time if the TLB has access time of 500ns and we observe a 80% TLB hit rate? Show your work.
EAT = (3us + 500ns)*0.8 + (6us + 500ns)*(1-0.8) = 4.1us
What is the effective access time if the TLB has access time of 800ns and we observe a 95% TLB hit rate? Show your work.
EAT = (3us + 800ns)* (6us + 800ns)*(1-0.95) = 3.95us

6. Homework 4 Q3
In the above diagram, what is the term for the phenomena occurring in region A?
Thrashing
What is one way to control the phenomena?
Establish an acceptable page-fault rate
Name one system other than paging which may exhibit the phenomena.
Cache, TLB, heap, process scheduling (anytime you spend more time swapping than using something)

7. Homework 4 Q3
Consider a file currently consisting of 100 blocks. Assume that the filecontrol block (and the index block, in the case of indexed allocation) is already in memory. Calculate how many disk I/O operations are required for contiguous, linked, and indexed (single-level) allocation strategies, if, for one block, the following conditions hold. In the contiguous-allocation case, assume that there is no room to grow at the beginning but there is room to grow at the end. Also assume that the block information to be added is stored in memory.
Textbook section 12.4
You need to read something and then write it elsewhere to move it
Doing things in memory is ‘free’

8. Homework 4 Q3
Credit: Abraham_Silberschatz-Operating_System_Concepts__9t

9. Homework 4 Q3 a. The block is added at the beginning.
Consider a file currently consisting of 100 blocks. Assume that the filecontrol block (and the index block, in the case of indexed allocation) is already in memory. Calculate how many disk I/O operations are required for contiguous, linked, and indexed (single-level) allocation strategies, if, for one block, the following conditions hold. In the contiguous-allocation case, assume that there is no room to grow at the beginning but there is room to grow at the end. Also assume that the block information to be added is stored in memory.
a. The block is added at the beginning.
b. The block is added in the middle.
c. The block is added at the end.
d. The block is removed from the beginning.
e. The block is removed from the middle.
f. The block is removed from the end.
Answer:
Contiguous Linked Indexed a b c d e f

10. Homework 4 Q5
Limit?

11. Homework 4 Q5 STBR 111010 Memory address contents note 10 110 S1PD 11
1000
100
1010
101
1100
1110
S1PT1
111
10000
10010
S1PT2
1001
10100
10110
S1PT3
1011
11000
11010
S1PT4
1101
11100
S1F1
1111
S1F2
10001
101001
10010
101010
S1F3
10011
10100
11001
S1F4
10101
10110
111100
S1F5
10111
11000
S1F6
11010
S1F7
11011
11100
S1F8
11101
11110
100010
S2PD
11111
100100
100000
100110
100001
101000
101010
S2PT1
100011
101100
101110
S2PT2
100101
110000
110010
S2PT3
100111
110100
110110
S2PT4
101001
111000
S2F1
101011
S2F2
101101
101110
S2F3
101111
100000
110000
S2F4
110001
110010
S2F5
110011
110100
S2F6
110101
11011
110110
S2F7
110111
111000
S2F8
111001
111010 10
seg table
111011
11110

12. Homework 4 Q5 Virtual Address Value 01010101 (example) 01110001
101001

13. Topics to review for the final
Everything from the review lecture
Homework problems

14. Any questions?
Good luck on the final