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 | ||||||
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 | ||||||
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 | ||||||
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 | ||||||
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 |
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 | ||||||
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 | ||||||
P3 | 2ms | P3 | 0ms | 1ms | 1ms | |
P4 | 3ms | P3, P4 | 0ms | 2ms | 2ms | |
P5 | 4ms | P3, P4, P5 | 0ms | 5ms | 5ms | |
7-8ms | ||||||
P4 | 3ms | P4 | 0ms | 2ms | 2ms | |
P5 | 4ms | P4, P5 | 0ms | 5ms | 5ms | |
8-10ms | ||||||
P5 | 4ms | P5 | 0ms | 5ms | 5ms | |
10-15ms |
Gantt Chart for Above Execution
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++ 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 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 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# 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 |
// 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(); |
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.