An OS follows round-robin scheduling with time quantum of 4ms. Assuming that the CPU is free now and there are 20 processes waiting in the ready queue, the maximum amount of time that a process waits before getting into the CPU is _________.
76 ms
Analyzing Round-Robin Scheduling and Waiting Time
The question asks about the maximum amount of time a process waits before getting into the CPU for the first time in a system using Round-Robin scheduling. We are given the time quantum, the number of processes initially in the ready queue, and that the CPU is initially free.
In Round-Robin scheduling, each process in the ready queue gets a turn to use the CPU for a fixed amount of time, called the time quantum. After a process uses the CPU for one quantum (or finishes sooner), it is moved to the end of the ready queue if it hasn't finished execution, and the next process in the queue gets its turn. This cycle repeats.
Calculating Maximum Initial Waiting Time
We have 20 processes waiting in the ready queue when the CPU becomes free. Let's label them P1, P2, ..., P20, where P1 is at the front of the queue and P20 is at the end.
The CPU is free, so P1 starts executing immediately. It runs for one time quantum.
- P1 runs for 4 ms.
- After P1 finishes its quantum, P2 gets the CPU. P2 runs for 4 ms.
- After P2 finishes its quantum, P3 gets the CPU. P3 runs for 4 ms.
- This continues for all the processes initially ahead of a particular process in the queue.
A process experiences the maximum waiting time *before getting into the CPU for the first time* if it is the last process in the ready queue when the scheduling begins. In this case, the process P20 is at the end of the initial queue.
For P20 to get the CPU for the first time, all 19 processes (P1 through P19) that were initially ahead of it must complete their first time quantum.
The time elapsed before P20 gets the CPU is the sum of the time quanta of the 19 processes that ran before it.
Number of processes ahead of P20 = Total processes - 1 = $20 - 1 = 19$
Time quantum = $4$ ms
Maximum waiting time for the first CPU access = (Number of processes ahead) $\times$ (Time quantum)
Maximum waiting time = $19 \times 4 \text{ ms}$
Maximum waiting time = $76 \text{ ms}$
Therefore, the maximum amount of time a process waits before getting into the CPU for the first time is 76 ms.
Revision Table: Round-Robin Concepts
| Concept | Description |
|---|---|
| Round-Robin Scheduling | A CPU scheduling algorithm where each process is assigned a fixed time unit (quantum) in a cyclic way. |
| Time Quantum | A small unit of time assigned to each process for execution in Round-Robin scheduling. |
| Ready Queue | A queue holding processes residing in main memory, ready and waiting to execute. |
| Waiting Time | The total time a process spends waiting in the ready queue. The calculation in the question is specifically for the first waiting period before initial CPU access. |
Additional Information: Understanding Waiting Time
It's important to distinguish between the initial waiting time (before the first CPU burst) and the total waiting time for a process.
- Initial Waiting Time: This is the time from when a process enters the ready queue until it first starts executing on the CPU. This is what was calculated in the problem.
- Total Waiting Time: This is the sum of all the time periods a process spends waiting in the ready queue after its first CPU burst (if it was preempted) until it finishes execution. A process might get preempted multiple times in Round-Robin and wait in the queue again.
The Round-Robin algorithm is designed to provide fairness and prevent starvation by giving each process a chance to run regularly. The choice of time quantum is critical; a very small quantum can lead to high context switching overhead, while a large quantum can make Round-Robin behave like First-Come, First-Served (FCFS).
Important Questions from Process Scheduling
Which of the following queues keeps a set of all processes residing in main memory ready and waiting to execute?
- Match the LIST-I with LIST-IIChoose the correct answer from the options given below:
LIST-I LIST-II A. Short term scheduler I. Determine which process are added to the system for execution level place them in ready queue. B. Medium term scheduler II. Select which process should be executed next and allocate CPU time. C. Long term scheduler III. Manage the degree of multi programming by swapping processes between main memory and disk storage D. I/O Scheduler IV. Order the input/output requests from processes waiting for I/O operations.
