Operating Systems | Set 13

3

Following questions have been asked in GATE CS 2007 exam.

1) A virtual memory system uses First In First Out (FIFO) page replacement policy and allocates a fixed number of frames to a process. Consider the following statements:
P: Increasing the number of page frames allocated to a process sometimes increases the page fault rate.
Q: Some programs do not exhibit locality of reference. Which one of the following is TRUE?

(A) Both P and Q are true, and Q is the reason for P
(B) Both P and Q are true, but Q is not the reason for P.
(C) P is false, but Q is true
(D) Both P and Q are false.

Answer (B)
P is true. Increasing the number of page frames allocated to process may increases the no. of page faults (See Belady’s Anomaly).
Q is also true, but Q is not the reason for-P as Belady’s Anomaly occurs for some specific patterns of page references.



2) A single processor system has three resource types X, Y and Z, which are shared by three processes. There are 5 units of each resource type. Consider the following scenario, where the column alloc denotes the number of units of each resource type allocated to each process, and the column request denotes the number of units of each resource type requested by a process in order to complete execution. Which of these processes will finish LAST?

 
    alloc           request
    X Y Z            X Y Z
P0  1 2 1            1 0 3
P1  2 0 1            0 1 2
P2  2 2 1            1 2 0

(A) P0
(B) P1
(C) P2
(D) None of the above, since the system is in a deadlock

Answer (C)
Once all resources (5, 4 and 3 instances of X, Y and Z respectively) are allocated, 0, 1 and 2 instances of X, Y and Z are left. Only needs of P1 can be satisfied. So P1 can finish its execution first. Once P1 is done, it releases 2, 1 and 3 units of X, Y and Z respectively. Among P0 and P2, needs of P0 can only be satisfied. So P0 finishes its execution. Finally, P2 finishes its execution.



3) Two processes, P1 and P2, need to access a critical section of code. Consider the following synchronization construct used by the processes:Here, wants1 and wants2 are shared variables, which are initialized to false. Which one of the following statements is TRUE about the above construct?

  /* P1 */
while (true) {
  wants1 = true;
  while (wants2 == true);
  /* Critical
    Section */
  wants1=false;
}
/* Remainder section */       


/* P2 */
while (true) {
  wants2 = true;
  while (wants1==true);
  /* Critical
    Section */
  wants2 = false;
}
/* Remainder section */

(A) It does not ensure mutual exclusion.
(B) It does not ensure bounded waiting.
(C) It requires that processes enter the critical section in strict alternation.
(D) It does not prevent deadlocks, but ensures mutual exclusion.

Answer (D)

The above synchronization constructs don’t prevent deadlock. When both wants1 and wants2 become true, both P1 and P2 stuck forever in their while loops waiting for each other to finish.



4) Consider the following statements about user level threads and kernel level threads. Which one of the following statement is FALSE?
(A) Context switch time is longer for kernel level threads than for user level threads.
(B) User level threads do not need any hardware support.
(C) Related kernel level threads can be scheduled on different processors in a multi-processor system.
(D) Blocking one kernel level thread blocks all related threads.

Answer (D)
Since kernel level threads are managed by kernel, blocking one thread doesn’t cause all related threads to block. It’s a problem with user level threads. See this for more details.

Please see GATE Corner for all previous year paper/solutions/explanations, syllabus, important dates, notes, etc.

Please write comments if you find any of the answers/explanations incorrect, or you want to share more information about the topics discussed above

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