Operating system GATE CS PYQ QUIZ (original) (raw)

Consider the virtual page reference string 1, 2, 3, 2, 4, 1, 3, 2, 4, 1 On a demand paged virtual memory system running on a computer system that main memory size of 3 pages frames which are initially empty. Let LRU, FIFO and OPTIMAL denote the number of page faults under the corresponding page replacements policy. Then

A file system with 300 GByte disk uses a file descriptor with 8 direct block addresses, 1 indirect block address and 1 doubly indirect block address. The size of each disk block is 128 Bytes and the size of each disk block address is 8 Bytes. The maximum possible file size in this file system is

• Dependent on the size of the disk

Consider the 3 processes, P1, P2 and P3 shown in the table.

Process Arrival time Time Units Required
P1 0 5
P2 1 7
P3 3 4

The completion order of the 3 processes under the policies FCFS and RR2 (round robin scheduling with CPU quantum of 2 time units) are

• FCFS: P1, P2, P3
  RR2: P1, P2, P3
• FCFS: P1, P3, P2
  RR2: P1, P3, P2
• FCFS: P1, P2, P3
  RR2: P1, P3, P2
• FCFS: P1, P3, P2
  RR2: P1, P2, P3

Fetch_And_Add(X,i) is an atomic Read-Modify-Write instruction that reads the value of memory location X, increments it by the value i, and returns the old value of X. It is used in the pseudocode shown below to implement a busy-wait lock. L is an unsigned integer shared variable initialized to 0. The value of 0 corresponds to lock being available, while any non-zero value corresponds to the lock being not available.

AcquireLock(L){
while (Fetch_And_Add(L,1))
L = 1;
}
ReleaseLock(L){
L = 0;
}

This implementation

• fails as L can take on a non-zero value when the lock is actually available
• works correctly but may starve some processes
• works correctly without starvation

A process executes the code

fork();
fork();
fork();

The total number of child processes created is

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….?

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.

Consider the following statements about process state transitions for a system using preemptive scheduling.

• **I. A running process can move to ready state.
• **II. A ready process can move to running state.
• **III. A blocked process can move to running state.
• **IV. A blocked process can move to ready state.

Which of the above statements are TRUE ?

Consider a non-negative counting semaphore S. The operation P(S) decrements S, and V(S) increments S. During an execution, 20 P(S) operations and 12 V(S) operations are issued in some order. The largest initial value of S for which at least one P(S) operation will remain blocked is ________.

Consider the following two-process synchronization solution.

**Process 0 **Process 1

Entry: loop while (turn == 1); Entry: loop while (turn == 0);
(critical section) (critical section)
Exit: turn = 1; Exit turn = 0;

The shared variable turn is initialized to zero. Which one of the following is TRUE?

• This is a correct two-process synchronization solution.
• This solution violates mutual exclusion requirement.
• This solution violates progress requirement.
• This solution violates bounded wait requirement.

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