first come first serve detail explanation

FCFS Scheduling Algorithm Questions with step-by-step solution + theory

What Is FCFS Scheduling Algorithm in Operating System ?

FCFS stands for (First Come First Serve), is one of the simplest scheduling algorithms in operating system. It schedules processes based on their arrival time.The process Which comes first according to the arrival time executes first and all other process wait in a queue untill the running process completes its work.

Method to Solve FCFS Scheduling Algorithm

  • Step 1: List all the processes and their arrival times in the order they arrive.
  • Step 2: Assign CPU to the process that arrives first and continue until it completes execution.
  • Step 3: After completion, check if the next process is ready to execute.
  • Step 4: Repeat the process for all other processes in the queue based on arrival time.
  • Step 5: Calculate the waiting time for each process as:
    • Waiting Time (WT) = Turn Around Time (TAT) - Burst Time (BT)
  • Step 6: Calculate the turnaround time for each process as:
  • Step 7: Sum up the total waiting time and turnaround time for performance evaluation.

Advantages and Disadvantages of FCFS Scheduling

Advantages Disadvantages
Simple and easy to implement. Long waiting times for processes with long burst times ( convoy effect ).
No starvation, as every process gets a turn. Poor response time for time-critical processes.
Fair for long-running processes. Not optimal for a multi-user or real-time system.

Complete List of Formulas to Solve FCFS Scheduling Algorithm Problems

  1. Turn Around Time (TAT) = Completion Time (CT) - Arrival Time (AT)
  2. Waiting Time (WT) = Turn Around Time (TAT) - Burst Time (BT)
  3. Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  4. Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

This set includes 17 questions based on the First Come First Served (FCFS) process scheduling algorithm. Each problem is organized in a tabular format and comes with a detailed solution, including the Gantt chart, average turnaround time (TAT), and average waiting time (WT).

Exercise 1 : Calculate TAT, WT, and Completion Time Using FCFS Scheduling (Based on the Table Below)

Process Arrival Time Burst Time
P1 0 2
P2 2 4
P3 4 3
P4 5 1

Solution

Gantt Chart

first-come-first-serve(fcfs) | Question 1 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 2 2 0
P2 6 4 0
P3 9 5 2
P4 10 5 4

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (2 + 4 + 5 + 5) / 4 = 16 / 4 = 4 units
Average Waiting Time = (0 + 0 + 2 + 4) / 4 = 6 / 4 = 1.5 units

Exercise 2 : Determine Average TAT, WT, and Completion Time Using FCFS Algorithm with Given Processes

Process Arrival Time Burst Time
P1 1 5
P2 2 3
P3 3 6

Solution

Gantt Chart

first-come-first-serve (fcfs) | Question 1 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 6 5 0
P2 9 7 4
P3 15 12 6

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (5 + 7 + 12) / 3 = 24 / 3 = 8 units
Average Waiting Time = (0 + 4 + 6) / 3 = 10 / 3 = 3.33 units

Exercise 3 : Given 4 processes with arrival and burst times, calculate the Gantt chart, waiting time, and turnaround time using FCFS?

Process Arrival Time Burst Time
P1 1 2
P2 3 4
P3 5 3
P4 6 2

Solution

Gantt Chart

first-come-first-serve (fcfs) | Question 2 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 3 2 0
P2 7 4 0
P3 10 5 2
P4 12 6 4

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (2 + 4 + 5 + 6) / 4 = 17 / 4 = 4.25 units
Average Waiting Time = (0 + 0 + 2 + 4) / 4 = 6 / 4 = 1.5 units

Exercise 4 : find the average Turn Around Time and Waiting Time of following processes using FCFS(FIRST COME FIRST SERVED) process scheduling algorithm?

Process Arrival Time Burst Time
P1 0 4
P2 1 3
P3 2 1
P4 3 2

Solution

Gantt Chart

first-come-first-serve (fcfs) | Question 4 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 4 4 0
P2 7 6 3
P3 8 6 5
P4 10 7 5

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (4 + 6 + 6 + 7) / 4 = 23 / 4 = 5.75 units
Average Waiting Time = (0 + 3 + 5 + 5) / 4 = 13 / 4 = 3.25 units

Exercise 5 : Given 4 processes with arrival and burst times, calculate the Gantt chart waiting time and turnaround time using FCFS

Process Arrival Time Burst Time
P1 2 3
P2 4 1
P3 6 2
P4 7 4

Solution

Gantt Chart

first-come-first-serve (fcfs) | Question 5 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 5 3 0
P2 6 2 1
P3 8 2 0
P4 12 5 1

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (3 + 2 + 2 + 5) / 4 = 12 / 4 = 3 units
Average Waiting Time = (0 + 1 + 0 + 1) / 4 = 2 / 4 = 0.5 units

Exercise 6 : find the average Turn Around Time and Waiting Time of following processes using FCFS(FIRST COME FIRST SERVED) process scheduling algorithm?

Process Arrival Time Burst Time
P1 0 6
P2 2 3
P3 4 5
P4 6 2

Solution

Gantt Chart

first-come-first-serve (fcfs) | Question 6 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 6 6 0
P2 9 7 4
P3 14 10 5
P4 16 10 8

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (6 + 7 + 10 + 10) / 4 = 33 / 4 = 8.25 units
Average Waiting Time = (0 + 4 + 5 + 8) / 4 = 17 / 4 = 4.25 units

Exercise 7 find the average Turn Around Time and Waiting Time of following processes using FCFS(FIRST COME FIRST SERVED) process scheduling algorithm?

Process Arrival Time Burst Time
P1 0 7
P2 1 5
P3 2 3

Solution

Gantt Chart

first-come-first-serve (fcfs) | Question 7 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 7 7 0
P2 12 11 6
P3 15 13 10

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (7 + 11 + 13) / 3 = 31 / 3 = 10.33 units
Average Waiting Time = (0 + 6 + 10) / 3 = 16 / 3 = 5.33 units

Exercise 8 : find the average Turn Around Time and Waiting Time of following processes using FCFS(FIRST COME FIRST SERVED) process scheduling algorithm?

Process Arrival Time Burst Time
P1 3 2
P2 5 4
P3 7 3
P4 9 1

Solution

Gantt Chart

first-come-first-serve (fcfs) | Question 1 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 5 2 0
P2 9 4 0
P3 12 5 2
P4 13 4 3

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (2 + 4 + 5 + 4) / 4 = 15 / 4 = 3.75 units
Average Waiting Time = (0 + 0 + 2 + 3) / 4 = 5 / 4 = 1.25 units

Exercise 9 : find the average Turn Around Time and Waiting Time of following processes using FCFS(FIRST COME FIRST SERVED) process scheduling algorithm?

Process Arrival Time Burst Time
P1 0 4
P2 1 5
P3 3 3
P4 5 2

Solution

Gantt Chart

first-come-first-serve (fcfs) | Question 9 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 4 4 0
P2 9 8 3
P3 12 9 6
P4 14 9 7

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (4 + 8 + 9 + 9) / 4 = 30 / 4 = 7.5 units
Average Waiting Time = (0 + 3 + 6 + 7) / 4 = 16 / 4 = 4 units

Question 10 : Consider 3 processes arriving at time intervals P1, P2, P3. Compute the average waiting time and turnaround time using FCFS scheduling?

Process Arrival Time Burst Time
P1 0 4
P2 2 3
P3 4 2

Solution

Gantt Chart

first-come-first-serve (fcfs) | Question 10 Solution | Gantt Chart Part 1
Process Completion Time Turn Around Time Waiting Time
P1 4 4 0
P2 7 5 2
P3 9 5 3

Formulas:

  • Average Turn Around Time (Average TAT) = Σ(TAT) / Number of Processes
  • Average Waiting Time (Average WT) = Σ(WT) / Number of Processes

Average Turn Around Time = (4 + 5 + 5) / 3 = 14 / 3 = 4.67 units
Average Waiting Time = (0 + 2 + 3) / 3 = 5 / 3 = 1.67 units

List of books that help you learn the first come first serve(FCFS) scheduling algorithm

List of books that help you learn the first come first serve (FCFS) scheduling algorithm