Round Robin Scheduling (Turnaround Time & Waiting Time)

Summary

This lecture explains round-robin scheduling, focusing on calculating turnaround and waiting times. It uses a CPU example with three processes (P1, P2, P3), each with different burst times, and a time quantum of 4 milliseconds. The lecture demonstrates how to form a Gantt chart, then calculates the turnaround and waiting times using two methods. The first method uses formulas based on completion and arrival times, while the second method calculates waiting times based on last start time, preemption, and time quantum. The lecture concludes with calculating average turnaround and waiting times, emphasizing the importance of balancing time quantum for efficiency.

Takeaways

• 🔄 Round-robin scheduling is a process scheduling algorithm that assigns each process a fixed time slot or quantum to use the CPU.
• 🕒 In this example, the time quantum is set to 4 milliseconds, meaning each process is allowed to execute for 4 milliseconds before being preempted.
• 📈 The Gantt chart is a key tool for visualizing how processes are executed over time in a round-robin scheduling system.
• 📊 Turnaround time is calculated as the completion time minus the arrival time, and waiting time is the turnaround time minus the burst time.
• 📉 Two methods are presented for calculating waiting times: one that calculates both turnaround and waiting times, and another that focuses solely on waiting times.
• 🔢 The formula for calculating waiting time when only needing the waiting time is: Waiting Time = Last Start Time - Arrival Time - (Preemption * Time Quantum).
• 📋 It's important to note that processes that are preempted are put back at the end of the queue for their next turn.
• 🔄 The script uses an example with three processes (P1, P2, P3) with burst times of 24ms, 3ms, and 3ms respectively, all arriving at time 0.
• 📊 The average turnaround time for the given processes is calculated to be 15.67 milliseconds, and the average waiting time is 5.67 milliseconds.
• ⚖️ Striking a balance with the time quantum is crucial for efficiency; too long makes it similar to FCFS, too short leads to excessive context switching.

Q & A

What is round-robin scheduling?

Round-robin scheduling is a CPU scheduling algorithm where each process is given a fixed time slot or quantum to use the CPU, known as a time quantum. If a process does not complete within its time quantum, it is preempted, and the CPU is assigned to the next process in the queue.

What is the time quantum in the context of round-robin scheduling?

The time quantum is the maximum amount of time that a process can use the CPU before being preempted. In the given example, the time quantum is set to 4 milliseconds.

How does the preemption of processes work in round-robin scheduling?

In round-robin scheduling, when a process has used up its time quantum, it is preempted, and the CPU is given to the next process in the queue. This continues in a cyclic manner until all processes have completed their execution.

What is a Gantt chart and why is it used in calculating turnaround and waiting times?

A Gantt chart is a graphical representation of the timeline for a set of processes. It is used in calculating turnaround and waiting times because it visually shows the execution and waiting periods of each process, which helps in determining when each process starts and finishes.

How is turnaround time calculated in the context of round-robin scheduling?

Turnaround time is calculated by subtracting the arrival time from the completion time of a process. This gives the total time a process has spent in the system from its arrival to its completion.

What is waiting time in the context of process scheduling?

Waiting time is the amount of time a process spends waiting in the queue before it starts executing. It is calculated by subtracting the burst time from the turnaround time.

What are the two methods mentioned in the script for calculating waiting times in round-robin scheduling?

The two methods are: 1) Using the formula where waiting time equals turnaround time minus burst time, which is useful for calculating both turnaround times and waiting times. 2) Using the formula where waiting time equals last start time minus arrival time minus (preemption times time quantum), which is useful for calculating only waiting times.

How is the average turnaround time calculated for a set of processes?

The average turnaround time is calculated by summing the turnaround times of all processes and dividing by the number of processes. In the script, it is calculated as (30 + 7 + 10) / 3, which equals 15.66 milliseconds.

How is the average waiting time calculated for a set of processes?

The average waiting time is calculated by summing the waiting times of all processes and dividing by the number of processes. In the script, it is calculated as (6 + 4 + 7) / 3, which equals 5.66 milliseconds.

Why is it important to balance the time quantum in round-robin scheduling?

Balancing the time quantum is crucial because if it's too long, the scheduling algorithm starts to resemble first-come first-serve, losing the benefits of time sharing. If it's too short, there will be too many context switches, leading to inefficiency. The right balance ensures fairness and efficiency in process execution.

How does the script illustrate the execution of processes in round-robin scheduling?

The script illustrates the execution of processes by detailing the Gantt chart formation and the execution sequence of processes P1, P2, and P3 with their respective burst times and how they are interrupted by the time quantum and queued for further execution.