#12 First Come First Serve (FCFS) Scheduling Algorithm Explained | Non-Preemptive CPU Scheduling

Summary

In this video, the First Come First Serve (FCFS) scheduling algorithm is explained in detail. This non-preemptive scheduling method ensures that the process which arrives first is executed first, regardless of its burst time. The video covers how to calculate important metrics such as completion time, turnaround time, and waiting time using a Gantt chart. It also discusses the throughput, which measures the number of processes completed in a given time interval. By walking through an example with five processes, viewers learn the step-by-step process of applying FCFS to compute average times and understand its performance in an operating system.

Takeaways

• 😀 FCFS (First-Come, First-Served) is a non-preemptive scheduling algorithm where processes are executed in the order they arrive without interruption.
• 😀 In FCFS, the process with the earliest arrival time is executed first, regardless of its processing time.
• 😀 The algorithm processes each task in the ready queue sequentially, ensuring that no task is interrupted once it begins execution.
• 😀 FCFS scheduling involves creating a Gantt chart to visually represent the execution timeline of processes.
• 😀 To calculate completion time, subtract the arrival time from the completion time of each process.
• 😀 Turnaround time is calculated by subtracting the arrival time from the completion time of a process, representing how long the process stays in the system.
• 😀 Waiting time is the difference between the turnaround time and the burst time of a process, showing how long a process waited before execution.
• 😀 The average completion time is calculated by dividing the sum of all completion times by the number of processes.
• 😀 Average turnaround time is the sum of all turnaround times divided by the number of processes.
• 😀 Throughput in FCFS scheduling is calculated as the number of processes divided by the difference between the maximum completion time and the minimum arrival time.
• 😀 FCFS is simple and easy to implement but may not be efficient in terms of average waiting time, as processes with longer burst times can delay the execution of shorter processes.

Q & A

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

FCFS is a non-preemptive scheduling algorithm where processes are executed in the order they arrive in the ready queue, without being interrupted. The process that arrives first is executed first, regardless of its processing time.

What does 'non-preemptive' mean in the context of FCFS scheduling?

'Non-preemptive' means that once a process starts executing, it will run to completion without being interrupted by other processes, even if they arrive later.

How does FCFS decide which process to execute first?

FCFS decides based on the arrival time of the processes. The process that arrives first is executed first, irrespective of its burst time (processing time).

What is the role of burst time in FCFS scheduling?

In FCFS scheduling, burst time (processing time) does not affect the order of execution. The process that arrives first is executed first, regardless of how long it takes to complete.

How do you calculate the completion time for a process in FCFS?

The completion time for a process is the time when it finishes execution. It can be determined by adding the burst time of each process to the current time as processes execute in sequence.

What is the formula for calculating the turnaround time of a process?

Turnaround time is calculated as the difference between the completion time and the arrival time of a process: Turnaround Time = Completion Time - Arrival Time.

How is waiting time calculated in FCFS scheduling?

Waiting time is the time a process spends in the ready queue before it starts execution. It is calculated as the difference between turnaround time and burst time: Waiting Time = Turnaround Time - Burst Time.

What is the average completion time, and how is it calculated?

The average completion time is the mean of all the completion times of the processes. It is calculated by summing the completion times of all processes and dividing by the total number of processes.

What is throughput, and how is it calculated in FCFS scheduling?

Throughput is the number of processes completed per unit of time. It is calculated as the number of processes divided by the difference between the maximum completion time and the minimum arrival time: Throughput = Number of Processes / (Max Completion Time - Min Arrival Time).

What is the significance of a Gantt chart in calculating the various times in FCFS scheduling?

A Gantt chart visually represents the execution of processes in FCFS scheduling. It helps to clearly show the order in which processes execute, allowing for easy calculation of completion times, turnaround times, and waiting times.