GATE CS 2021 Set 2 Q14 — CPU Scheduling

Let us analyze each statement individually to determine its correctness:

• Statement A: Turnaround time includes waiting time
  Correct. Turnaround Time ($TAT$) is the total time spent by a process in the system from its arrival to its completion. It is calculated as: $$\text{Turnaround Time} = \text{Completion Time} - \text{Arrival Time}$$ Alternatively, it can be expressed as: $$\text{Turnaround Time} = \text{Burst Time} + \text{Waiting Time} + \text{I/O Time}$$ Hence, turnaround time directly includes waiting time.


• Statement B: The goal is to only maximize $\text{CPU}$ utilization and minimize throughput
  Incorrect. The objective of efficient CPU scheduling is to maximize both CPU utilization and throughput (the number of processes completed per unit of time), while minimizing turnaround time, waiting time, and response time.


• Statement C: Round-robin policy can be used even when the $\text{CPU}$ time required by each of the processes is not known apriori
  Correct. Round-Robin (RR) scheduling allocates the CPU to processes in a cyclic order for a fixed time quantum. It does not require any prior knowledge of the processes' CPU burst times, unlike algorithms like Shortest Job First (SJF) or Shortest Remaining Time First (SRTF).


• Statement D: Implementing preemptive scheduling needs hardware support
  Correct. Preemptive scheduling requires a mechanism to interrupt a currently running process after a certain interval (e.g., when its time quantum expires). This is implemented using a hardware timer interrupt that periodically transfers control back to the operating system's scheduler. Without this hardware support, a process could run indefinitely unless it voluntarily yields the CPU.