OS Process Management

Question 111

Three processes arrive at time zero with CPU bursts of 16, 20 and 10 milliseconds. If the scheduler has prior knowledge about the length of the CPU bursts, the minimum achievable average waiting time for these three processes in a non-preemptive scheduler (rounded to nearest integer) is _____________ milliseconds.

• 12
• 36
• 46
• 10

Question 112

At a particular time of computation, the value of a counting semaphore is 7. Then 20 P(wait) operations and 15 V(signal) operations are completed on this semaphore. What is the resulting value of the semaphore?

• 28
• 12
• 2
• 42

Question 113

Processes P₁ and P₂ use critical_flag in the following routine to achieve mutual exclusion. Assume that critical_flag is initialized to FALSE in the main program.
get_exclusive_access ( ) { if (critical _flag == FALSE) { critical_flag = TRUE ; critical_region () ; critical_flag = FALSE; } } Consider the following statements.
i. It is possible for both P₁ and P₂ to access critical_region concurrently.
ii. This may lead to a deadlock.
Which of the following holds?

• (i) is false and (ii) is true
• Both (i) and (ii) are false
• (i) is true and (ii) is false
• Both (i) and (ii) are true

Question 114

Three concurrent processes X, Y, and Z execute three different code segments that access and update certain shared variables. Process X executes the P operation (i.e., wait) on semaphores a, b and c; process Y executes the P operation on semaphores b, c and d; process Z executes the P operation on semaphores c, d, and a before entering the respective code segments. After completing the execution of its code segment, each process invokes the V operation (i.e., signal) on its three semaphores. All semaphores are binary semaphores initialized to one. Which one of the following represents a deadlockfree order of invoking the P operations by the processes? (GATE CS 2013)

• X: P(a)P(b)P(c) Y:P(b)P(c)P(d) Z:P(c)P(d)P(a)

• X: P(b)P(a)P(c) Y:P(b)P(c)P(d) Z:P(a)P(c)P(d)

• X: P(b)P(a)P(c) Y:P(c)P(b)P(d) Z:P(a)P(c)P(d)

• X: P(a)P(b)P(c) Y:P(c)P(b)P(d) Z:P(c)P(d)P(a)

Question 115

Consider the following threads, T1, T2, and T3 executing on a single processor, synchronized using three binary semaphore variables, S1, S2, and S3, operated upon using standard wait() and signal(). The threads can be context switched in any order and at any time. 

Which initialization of the semaphores would print the sequence BCABCABCA….?

• S₁ = 1; S₂ = 1; S₃ = 1

• S₁ = 1; S₂ = 1; S₃ = 0

• S₁ = 1; S₂ = 0; S₃ = 0

• S₁ = 0; S₂ = 1; S₃ = 1