Operating Systems | Set 8

Following questions have been asked in GATE 2009 CS exam.

1) In which one of the following page replacement policies, Belady’s anomaly may occur?
(A) FIFO
(B) Optimal
(C) LRU
(D) MRU

Answer (A)
Belady’s anomaly proves that it is possible to have more page faults when increasing the number of page frames while using the First in First Out (FIFO) page replacement algorithm.
See the wiki page for an example of increasing page faults with number of page frames.

2) The essential content(s) in each entry of a page table is / are
(A) Virtual page number
(B) Page frame number
(C) Both virtual page number and page frame number
(D) Access right information

Answer (B)
A page table entry must contain Page frame number. Virtual page number is typically used as index in page table to get the corresponding page frame number. See this for details.

3) Consider a system with 4 types of resources R1 (3 units), R2 (2 units), R3 (3 units), R4 (2 units). A non-preemptive resource allocation policy is used. At any given instance, a request is not entertained if it cannot be completely satisfied. Three processes P1, P2, P3 request the sources as follows if executed independently.

Process P1: 
t=0: requests 2 units of R2 
t=1: requests 1 unit of R3 
t=3: requests 2 units of R1 
t=5: releases 1 unit of R2    
        and 1 unit of R1. 
t=7: releases 1 unit of R3 
t=8: requests 2 units of R4 
t=10: Finishes

Process P2: 
t=0: requests 2 units of R3 
t=2: requests 1 unit of R4 
t=4: requests 1 unit of R1 
t=6: releases 1 unit of R3 
t=8: Finishes

Process P3: 
t=0: requests 1 unit of R4 
t=2: requests 2 units of R1 
t=5: releases 2 units of R1 
t=7: requests 1 unit of R2 
t=8: requests 1 unit of R3 
t=9: Finishes

Which one of the following statements is TRUE if all three processes run concurrently starting at time t=0?
(A) All processes will finish without any deadlock
(B) Only P1 and P2 will be in deadlock.
(C) Only P1 and P3 will be in a deadlock.
(D) All three processes will be in deadlock

Answer (A)
We can apply the following Deadlock Detection algorithm and see that there is no process waiting indefinitely for a resource. See this for deadlock detection algorithm.

4) Consider a disk system with 100 cylinders. The requests to access the cylinders occur in following sequence:
4, 34, 10, 7, 19, 73, 2, 15, 6, 20
Assuming that the head is currently at cylinder 50, what is the time taken to satisfy all requests if it takes 1ms to move from one cylinder to adjacent one and shortest seek time first policy is used?
(A) 95ms
(B) 119ms
(C) 233ms
(D) 276ms

Answer (B)
4, 34, 10, 7, 19, 73, 2, 15, 6, 20
Since shortest seek time first policy is used, head will first move to 34. This move will cause 16*1 ms. After 34, head will move to 20 which will cause 14*1 ms. And so on. So cylinders are accessed in following order 34, 20, 19, 15, 10, 7, 6, 4, 2, 73 and total time will be (16 + 14 + 1 + 4 + 5 + 3 + 1 + 2 + 2 + 71)*1 = 119 ms.

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