Some processes may be in the waiting state, others may be in the running state and so on.

Have you ever thought how CPU selects one process out of some many processes for execution Yes, you got it right.

CPU uses some kind of process scheduling algorithms to select one process for its execution amongst so many processes.

The process scheduling algorithms are used to maximize CPU utilization by increasing throughput.

In this blog, we will learn about various process scheduling algorithms used by CPU to schedule a process.

But before starting this blog, if you are not familiar with Burst time, Arrival time, Exit time, Response time, Waiting time, Turnaround time, and Throughput, then you should first learn these topics by reading the blog from here.

Also, learn about Preemptive and Non-Preemptive scheduling from here.

Since you are done with the prerequisites of this blog, lets get started by learning different process scheduling algorithms.

First Come First Serve (FCFS) As the name suggests, the process coming first in the ready state will be executed first by the CPU irrespective of the burst time or the priority.

This is implemented by using the First In First Out (FIFO) queue.

So, what happens is that, when a process enters into the ready state, then the PCB of that process will be linked to the tail of the queue and the CPU starts executing the processes by taking the process from the head of the queue (learn more about PCB from here ).

If the CPU is allocated to a process then it cant be taken back until it finishes the execution of that process.

Example.

Average turnaround time: (743) 24.66ms Advantages of FCFS: It is the most simple scheduling algorithm and is easy to implement.

Disadvantages of FCFS: This algorithm is non-preemptive so you have to execute the process fully and after that other processes will be allowed to execute.

FCFS suffers from the Convey effect i.e.

Shortest Job First (Non-preemptive) In the FCFS, we saw if a process is having a very high burst time and it comes first then the other process with a very low burst time have to wait for its turn.

So, to remove this problem, we come with a new approach i.e.

Shortest Job First or SJF.

In this technique, the process having the minimum burst time at a particular instant of time will be executed first.

It is a non-preemptive approach i.e.

The burst time of P1 is 5ms and that of P3 is 2ms.

So, amongst these two, the process P3 will be executed first because its burst time is less than P1.

P3 will be executed for 2ms.

Now, after 6ms, we have two processes with us i.e.

P1 and P4 (because we are at 6ms and P4 comes at 5ms).

Amongst these two, the process P4 is having a less burst time as compared to P1.

Average turnaround time: (234) 5.75ms Advantages of SJF (non-preemptive): Short processes will be executed first.

Disadvantages of SJF (non-preemptive): It may lead to starvation if only short burst time processes are coming in the ready state(learn more about starvation from here ).

Shortest Job First (Preemptive) This is the preemptive approach of the Shortest Job First algorithm.

Preemptive Priority Scheduling Example