First Come First Serve – CPU Scheduling (Non-Preemptive)

Simplest CPU scheduling algorithm that schedules according to arrival times of processes. The first come first serve scheduling algorithm states that the process that requests the CPU first is allocated the CPU first. It is implemented by using the FIFO queue. When a process enters the ready queue, its PCB is linked to the tail of the queue. When the CPU is free, it is allocated to the process at the head of the queue. The running process is then removed from the queue. FCFS is a non-preemptive scheduling algorithm.

Characteristics of FCFS

• FCFS supports non-preemptive and preemptive CPU scheduling algorithms.
• Tasks are always executed on a First-come, First-serve concept.
• FCFS is easy to implement and use.
• This algorithm is not very efficient in performance, and the wait time is quite high.

Algorithm for FCFS Scheduling

• The waiting time for the first process is 0 as it is executed first.
• The waiting time for the upcoming process can be calculated by:

wt[i] =  ( at[i – 1] + bt[i – 1] + wt[i – 1] ) – at[i]

where 

• wt[i] = waiting time of current process 
• at[i-1] = arrival time of previous process 
• bt[i-1] = burst time of previous process 
• wt[i-1] = waiting time of previous process 
• at[i] = arrival time of current process 

• The Average waiting time can be calculated by: 

Average Waiting Time = (sum of all waiting time)/(Number of processes)

Examples to Show Working of Non-Preemptive First come First Serve CPU Scheduling Algorithm

Example-1: Consider the following table of arrival time and burst time for five processes P1, P2, P3, P4 and P5. 

Processes	Arrival Time	Burst Time
P1	0	4
P2	1	3
P3	2	1
P4	3	2
P5	4	5

The First come First serve CPU Scheduling Algorithm will work on the basis of steps as mentioned below:

Step 0: At time = 0,

• The process begins with P1 
• As it has an arrival time 0

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
0-1ms	P1	0ms		1ms	4ms	3ms

Step 1: At time = 1,

• The process P2 arrives
• But process P1 still executing, 
• Thus, P2 is kept on a waiting table and waits for its execution.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
1-2ms	P1	0ms		1ms	3ms	2ms
	P2	1ms	P2	0ms	3ms	3ms

Step 3: At time = 2,

• The process P3 arrives and kept in a waiting queue 
• While process P1 is still executing as its burst time is 4.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
2-3ms	P1	0ms		1ms	2ms	1ms
	P2	1ms	P2	0ms	3ms	3ms
	P3	2ms	P2, P3	0ms	1ms	1ms

Step 4: At time = 3,

• The process P4 arrives and kept in the waiting queue 
• While process P1 is still executing as its burst time is 4

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
3-4ms	P1	0ms		1ms	1ms	0ms
	P2	1ms	P2	0ms	3ms	3ms
	P3	2ms	P2, P3	0ms	1ms	1ms
	P4	3ms	P2, P3, P4	0ms	2ms	2ms

Step 5: At time = 4,

• The process P1 completes its execution 
• Process P5 arrives in waiting queue while process P2 starts executing

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
4-5ms	P2	1ms		1ms	3ms	2ms
	P3	2ms	P3	0ms	1ms	1ms
	P4	3ms	P3, P4	0ms	2ms	2ms
	P5	4ms	P3, P4, P5	0ms	5ms	5ms

Step 6: At time = 5,

• The process P2 completes its execution 

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
5-7ms	P2	1ms		2ms	2ms	0ms
	P3	2ms	P3	0ms	1ms	1ms
	P4	3ms	P3, P4	0ms	2ms	2ms
	P5	4ms	P3, P4, P5	0ms	5ms	5ms

Step 7: At time = 7,

• Process P3 starts executing, it has burst time of 1 thus, it completes execution at time interval 8

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
7-8ms	P3	2ms		1ms	1ms	0ms
	P4	3ms	P4	0ms	2ms	2ms
	P5	4ms	P4, P5	0ms	5ms	5ms

Step 8: At time 8,

• The process of P3 completes its execution 
• Process P4 starts executing, it has burst time of 2 thus, it completes execution at time interval 10.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
8-10ms	P4	3ms		2ms	2ms	0ms
	P5	4ms	P5	0ms	5ms	5ms

Step 9: At time 10,

• The process P4 completes its execution 
• Process P5 starts executing, it has burst time of 5 thus, it completes execution at time interval 15.

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
10-15ms	P5	4ms		5ms	5ms	0ms

Step 10: At time 15,

• Process P5 will finish its execution.
• The overall execution of the processes will be as shown below:

Time Instance	Process	Arrival Time	Waiting Table	Execution Time	Initial Burst Time	Remaining Burst  Time
0-1ms	P1	0ms		1ms	4ms	3ms
1-2ms	P1	0ms		1ms	3ms	2ms
	P2	1ms	P2	0ms	3ms	3ms
2-3ms	P1	0ms		1ms	2ms	1ms
	P2	1ms	P2	0ms	3ms	3ms
	P3	2ms	P2, P3	0ms	1ms	1ms
3-4ms	P1	0ms		1ms	1ms	0ms
	P2	1ms	P2	0ms	3ms	3ms
	P3	2ms	P2, P3	0ms	1ms	1ms
	P4	3ms	P2, P3, P4	0ms	2ms	2ms
4-5ms	P2	1ms		1ms	3ms	2ms
	P3	2ms	P3	0ms	1ms	1ms
	P4	3ms	P3, P4	0ms	2ms	2ms
	P5	4ms	P3, P4, P5	0ms	5ms	5ms
5-7ms	P2	1ms		2ms	2ms	0ms
	P3	2ms	P3	0ms	1ms	1ms
	P4	3ms	P3, P4	0ms	2ms	2ms
	P5	4ms	P3, P4, P5	0ms	5ms	5ms
7-8ms	P3	2ms		1ms	1ms	0ms
	P4	3ms	P4	0ms	2ms	2ms
	P5	4ms	P4, P5	0ms	5ms	5ms
8-10ms	P4	3ms		2ms	2ms	0ms
	P5	4ms	P5	0ms	5ms	5ms
10-15ms	P5	4ms		5ms	5ms	0ms

Gantt Chart for Above Execution

Gantt chart for First come First serve Scheduling

Waiting Time = Start time – Arrival time

P1 = 0 – 0 = 0
P2 = 4 – 1 = 3
P3 = 7 – 2 = 5
P4 = 8 – 3 = 5
P5 = 10 – 4 = 6

Average waiting time = (0 + 3 + 5 + 5+ 6 )/ 5 = 19 / 5 = 3.8 

Program for First Come First Serve Algorithm

C++

// C++ program to Calculate Waiting
// Time for given Processes
#include <iostream>
using namespace std;

// Function to Calculate waiting time
// and average waiting time
void CalculateWaitingTime(int at[],
                          int bt[], int N)
{

    // Declare the array for waiting
    // time
    int wt[N];

    // Waiting time for first process
    // is 0
    wt[0] = 0;

    // Print waiting time process 1
    cout << "PN\t\tAT\t\t"
         << "BT\t\tWT\n\n";
    cout << "1"
         << "\t\t" << at[0] << "\t\t"
         << bt[0] << "\t\t" << wt[0] << endl;

    // Calculating waiting time for
    // each process from the given
    // formula
    for (int i = 1; i < 5; i++) {
        wt[i] = (at[i - 1] + bt[i - 1]
                 + wt[i - 1]) - at[i];

        // Print the waiting time for
        // each process
        cout << i + 1 << "\t\t" << at[i]
             << "\t\t" << bt[i] << "\t\t"
             << wt[i] << endl;
    }

    // Declare variable to calculate
    // average
    float average;
    float sum = 0;

    // Loop to calculate sum of all
    // waiting time
    for (int i = 0; i < 5; i++) {
        sum = sum + wt[i];
    }

    // Find average waiting time
    // by dividing it by no. of process
    average = sum / 5;

    // Print Average Waiting Time
    cout << "\nAverage waiting time = "
         << average;
}

// Driver code
int main()
{
    // Number of process
    int N = 5;

    // Array for Arrival time
    int at[] = { 0, 1, 2, 3, 4 };

    // Array for Burst Time
    int bt[] = { 4, 3, 1, 2, 5 };

    // Function call to find
    // waiting time
    CalculateWaitingTime(at, bt, N);
    return 0;
}
//this code is contributed by snehalsalokhe

Java

// Java program to Calculate Waiting
// Time for given Processes
class GFG
{

// Function to Calculate waiting time
// and average waiting time
static void CalculateWaitingTime(int at[],
                          int bt[], int N)
{

    // Declare the array for waiting
    // time
    int []wt = new int[N];

    // Waiting time for first process
    // is 0
    wt[0] = 0;

    // Print waiting time process 1
    System.out.print("P.No.\tArrival Time\t"
        + "Burst Time\tWaiting Time\n");
    System.out.print("1"
        + "\t\t" +  at[0]+ "\t\t"
         + bt[0]+ "\t\t" +  wt[0] +"\n");

    // Calculating waiting time for
    // each process from the given
    // formula
    for (int i = 1; i < 5; i++) {
        wt[i] = (at[i - 1] + bt[i - 1] + wt[i - 1]) - at[i];

        // Print the waiting time for
        // each process
        System.out.print(i + 1+ "\t\t" +  at[i]
            + "\t\t" +  bt[i]+ "\t\t"
             + wt[i] +"\n");
    }

    // Declare variable to calculate
    // average
    float average;
    float sum = 0;

    // Loop to calculate sum of all
    // waiting time
    for (int i = 0; i < 5; i++) {
        sum = sum + wt[i];
    }

    // Find average waiting time
    // by dividing it by no. of process
    average = sum / 5;

    // Print Average Waiting Time
    System.out.print("Average waiting time = "
         + average);
}

// Driver code
public static void main(String[] args)
{
    // Number of process
    int N = 5;

    // Array for Arrival time
    int at[] = { 0, 1, 2, 3, 4 };

    // Array for Burst Time
    int bt[] = { 4, 3, 1, 2, 5 };

    // Function call to find
    // waiting time
    CalculateWaitingTime(at, bt, N);
}
}

// This code is contributed by 29AjayKumar

Python3

# Python3 program to Calculate Waiting
# Time for given Processes

# Function to Calculate waiting time
# and average waiting time
def CalculateWaitingTime(at, bt, N):

    # Declare the array for waiting
    # time
    wt = [0]*N;

    # Waiting time for first process
    # is 0
    wt[0] = 0;

    # Print waiting time process 1
    print("P.No.\tArrival Time\t" , "Burst Time\tWaiting Time");
    print("1" , "\t\t" , at[0] , "\t\t" , bt[0] , "\t\t" , wt[0]);

    # Calculating waiting time for
    # each process from the given
    # formula
    for i in range(1,5):
        wt[i] = (at[i - 1] + bt[i - 1] + wt[i - 1]) - at[i];

        # Print the waiting time for
        # each process
        print(i + 1 , "\t\t" , at[i] , "\t\t" , bt[i] , "\t\t" , wt[i]);


    # Declare variable to calculate
    # average
    average = 0.0;
    sum = 0;

    # Loop to calculate sum of all
    # waiting time
    for i in range(5):
        sum = sum + wt[i];

    # Find average waiting time
    # by dividing it by no. of process
    average = sum / 5;

    # Print Average Waiting Time
    print("Average waiting time = " , average);


# Driver code
if __name__ == '__main__':
    # Number of process
    N = 5;

    # Array for Arrival time
    at = [ 0, 1, 2, 3, 4 ];

    # Array for Burst Time
    bt = [ 4, 3, 1, 2, 5 ];

    # Function call to find
    # waiting time
    CalculateWaitingTime(at, bt, N);

# This code is contributed by 29AjayKumar

C#

// C# program to Calculate Waiting
// Time for given Processes
using System;

class GFG
{

// Function to Calculate waiting time
// and average waiting time
static void CalculateWaitingTime(int []at,
                          int []bt, int N)
{

    // Declare the array for waiting
    // time
    int []wt = new int[N];

    // Waiting time for first process
    // is 0
    wt[0] = 0;

    // Print waiting time process 1
    Console.Write("P.No.\tArrival Time\t"
        + "Burst Time\tWaiting Time\n");
    Console.Write("1"
        + "\t\t" +  at[0]+ "\t\t"
         + bt[0]+ "\t\t" +  wt[0] +"\n");

    // Calculating waiting time for
    // each process from the given
    // formula
    for (int i = 1; i < 5; i++) {
        wt[i] = (at[i - 1] + bt[i - 1] + wt[i - 1]) - at[i];

        // Print the waiting time for
        // each process
        Console.Write(i + 1+ "\t\t" +  at[i]
            + "\t\t" +  bt[i]+ "\t\t"
             + wt[i] +"\n");
    }

    // Declare variable to calculate
    // average
    float average;
    float sum = 0;

    // Loop to calculate sum of all
    // waiting time
    for (int i = 0; i < 5; i++) {
        sum = sum + wt[i];
    }

    // Find average waiting time
    // by dividing it by no. of process
    average = sum / 5;

    // Print Average Waiting Time
    Console.Write("Average waiting time = "
         + average);
}

// Driver code
public static void Main(String[] args)
{
    // Number of process
    int N = 5;

    // Array for Arrival time
    int []at = { 0, 1, 2, 3, 4 };

    // Array for Burst Time
    int []bt = { 4, 3, 1, 2, 5 };

    // Function call to find
    // waiting time
    CalculateWaitingTime(at, bt, N);
}
}

// This code is contributed by 29AjayKumar

Javascript

// Function to calculate waiting time and average waiting time
function calculateWaitingTime(at, bt, n) {
  // Declare the array for waiting time
  let wt = new Array(n);

  // Waiting time for first process is 0
  wt[0] = 0;

  // Print waiting time for process 1
  console.log("PN\t\tAT\t\tBT\t\tWT\n\n");
  console.log(`1\t\t${at[0]}\t\t${bt[0]}\t\t${wt[0]}\n`);

  // Calculate waiting time for each process from the given formula
  for (let i = 1; i < n; i++) {
    wt[i] = (at[i - 1] + bt[i - 1] + wt[i - 1]) - at[i];

    // Print the waiting time for each process
    console.log(`${i + 1}\t\t${at[i]}\t\t${bt[i]}\t\t${wt[i]}\n`);
  }

  // Declare variable to calculate average
  let average;
  let sum = 0;

  // Loop to calculate sum of all waiting time
  for (let i = 0; i < n; i++) {
    sum = sum + wt[i];
  }

  // Find average waiting time by dividing it by no. of process
  average = sum / n;

  // Print Average Waiting Time
  console.log(`\nAverage waiting time = ${average}`);
}

// Driver code
function main() {
  // Number of processes
  let n = 5;

  // Array for arrival time
  let at = [0, 1, 2, 3, 4];

  // Array for burst time
  let bt = [4, 3, 1, 2, 5];

  // Function call to find waiting time
  calculateWaitingTime(at, bt, n);
}

// Call the main function
main();

Output

PN        AT        BT        WT

1        0        4        0
2        1        3        3
3        2        1        5
4        3        2        5
5        4        5        6

Average waiting time = 3.8

Complexity Analysis:

• Time Complexity: O(N)
• Auxiliary Space: O(N)

Advantages of FCFS

• The simplest and basic form of CPU Scheduling algorithm
• Easy to implement
• First come first serve method
• It is well suited for batch systems where the longer time periods for each process are often acceptable.

Disadvantages of FCFS

• As it is a Non-preemptive CPU Scheduling Algorithm, hence it will run till it finishes the execution.
• The average waiting time in the FCFS is much higher than in the others
• It suffers from the Convoy effect.
• Not very efficient due to its simplicity
• Processes that are at the end of the queue, have to wait longer to finish.
• It is not suitable for time-sharing operating systems where each process should get the same amount of CPU time.