First Come First Served Scheduling (Solved Problem 1)

Summary

This lecture continues the discussion on the First-Come, First-Served (FCFS) scheduling algorithm, highlighting its disadvantages, such as the convoy effect, where short processes wait for longer ones to complete. The lecture demonstrates solving a problem with five processes, calculating their average waiting and turnaround times using the FCFS algorithm. Through detailed steps and Gantt chart analysis, it explains how to determine completion times, turnaround times, and waiting times, illustrating the potential inefficiencies of FCFS scheduling.

Takeaways

• 😀 First-Come First-Serve (FCFS) scheduling is one of the simplest scheduling algorithms.
• 😀 A major disadvantage of FCFS is the 'convoy effect,' where shorter processes wait for longer ones to finish.
• 😀 The lecture presents a problem involving five processes to demonstrate FCFS scheduling.
• 😀 The problem requires calculating average waiting time and average turnaround time using FCFS.
• 😀 The Gantt chart shows when each process gets the CPU and completes execution.
• 😀 In FCFS, the process that arrives first is served first, illustrated with process P3 arriving at time 0.
• 😀 Idle CPU time is represented by shaded boxes in the Gantt chart when no processes are waiting.
• 😀 Turnaround time is calculated as completion time minus arrival time.
• 😀 Waiting time is calculated as turnaround time minus burst time.
• 😀 The example problem results in an average turnaround time of 8 units and an average waiting time of 4.4 units.

Q & A

What is the First-Come-First-Serve (FCFS) scheduling algorithm?

FCFS is a simple scheduling algorithm where the process that arrives first is executed first. It operates in a queue-like fashion.

What is a major disadvantage of the FCFS scheduling algorithm?

A major disadvantage of FCFS is the 'convoy effect,' where processes with shorter burst times have to wait for longer processes to complete, resulting in higher average waiting and turnaround times.

What is the 'convoy effect' in FCFS scheduling?

The convoy effect occurs when shorter processes are forced to wait for longer processes to complete their execution, leading to inefficient CPU utilization and increased waiting times.

How is the Gantt chart used in FCFS scheduling?

A Gantt chart is used to visually represent the order and duration of processes being executed in FCFS scheduling, showing when each process gets the CPU and when it completes its execution.

What is the formula to calculate the turnaround time for a process?

The turnaround time is calculated as the completion time of the process minus its arrival time.

How is the waiting time for a process calculated in FCFS scheduling?

The waiting time is calculated as the turnaround time minus the burst time of the process.

Why did process P1 have a waiting time of 0 in the given example?

Process P1 had a waiting time of 0 because it arrived when the CPU was idle and immediately got the CPU for execution.

What are the arrival and burst times of the five processes in the given problem?

The arrival and burst times are as follows: P1 (arrival: 4, burst: 5), P2 (arrival: 6, burst: 4), P3 (arrival: 0, burst: 3), P4 (arrival: 6, burst: 2), P5 (arrival: 5, burst: 4).

How do you calculate the average turnaround time 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.

What was the average waiting time for the given set of processes?

The average waiting time for the given set of processes was 4.4 units of time.

How does the arrival time of processes affect the average turnaround and waiting times in FCFS scheduling?

The arrival time of processes can significantly affect the average turnaround and waiting times. If longer processes arrive before shorter ones, it can increase the average times due to the convoy effect.