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
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.