Operating System Tutorial

Operating System Tutorial Types of Operating System Evolution of Operating System Functions of Operating System Operating System Properties Operating System Services Components of Operating System Needs of the Operating System

Operating Systems

Linux Operating System Unix Operating System Ubuntu Operating System Chrome Operating Systems Fedora Operating System MAC Operating System MS Windows Operating System Solaris Operating System Cooperative Operating System CorelDRAW Operating System CentOS FreeBSD Operating Systems Batch Operating System MS-DOS Operating System Commercial Mobile Operating Systems


Difference Between Multi-programming and Multitasking Difference between C-LOOK and C-SCAN Difference between Rotational Latency and Disk Assess Time Trap vs Interrupt Difference between C-SCAN and SSTF Difference between SCAN and FCFS Difference between Seek Time and Disk Access Time Difference between SSTF and LOOK Difference between Process and Program in the Operating System Difference between Protection and Security in Operating System

How To

How to implement Monitors using Semaphores How to Install a Different Operating System on a PC


What is Kernel and Types of Kernel What is DOS Operating System What is Thread and Types of Thread What is Process Scheduler and Process Queue What is Context Switching What is CPU Scheduling What is Producer-Consumer Problem What is Semaphore in Operating System Monitors in Operating System What is Deadlock What is Paging and Segmentation What is Demand Paging What is Virtual Memory What is a Long term Scheduler What is Page Replacement in Operating System What is BSR Mode What is Convoy Effect What is Job Sequencing in Operating System Why is it critical for the Scheduler to distinguish between I/O-bound and CPU-bound programs Why is there a Need for an Operating System


Process Management Process State Scheduling Algorithm FCFS (First-come-First-Serve) Scheduling SJF (Shortest Job First) Scheduling Round-Robin CPU Scheduling Priority Based Scheduling HRRN (Highest Response Ratio Next) Scheduling Process Synchronization Lock Variable Mechanism TSL Mechanism Turn Variable Mechanism Interested Variable Mechanism Deadlock Avoidance Strategies for Handling Deadlock Deadlock Prevention Deadlock Detection and Recovery Resource Allocation Graph Banker’s Algorithm in Operating System Fixed Partitioning and Dynamic Partitioning Partitioning Algorithms Disk Scheduling Algorithms FCFS and SSTF Disk Scheduling Algorithm SCAN and C-SCAN Disk Scheduling Algorithm Look and C-Look Disk Scheduling Algorithm File in Operating System File Access Methods in Operating System File Allocation Method Directory Structure in Operating System N-Step-SCAN Disk Scheduling Feedback Queue in Operating System Contiguous Memory Allocation in Operating System Real-time Operating System Starvation in Operating System Thrashing in Operating System 5 Goals of Operating System Advantages of Operating System Advantages of UNIX Operating System Bit Vector in Operating System Booting Process in Operating System Can a Computer Run Without the Operating System Dining Philosophers Problem in Operating System Free Space Management in Operating System Inter Process Communication in Operating System Swapping in Operating System Memory Management in Operating System Multiprogramming Operating System Multitasking Operating Systems Multi-user Operating Systems Non-Contiguous Memory Allocation in Operating System Page Table in Operating System Process Scheduling in Operating System Segmentation in Operating System Simple Structure in Operating System Single-User Operating System Two Phase Locking Protocol Advantages and Disadvantages of Operating System Arithmetic operations in binary number system Assemblers in the operating system Bakery Algorithm in Operating System Benefits of Ubuntu Operating System CPU Scheduling Criteria in Operating System Critical Section in Operating System Device Management in Operating System Linux Scheduler in Operating System Long Term Scheduler in Operating System Mutex in Operating System Operating System Failure Peterson's Solution in Operating System Privileged and Non-Privileged Instructions in Operating System Swapping in Operating System Types of Operating System Zombie and Orphan Process in Operating System 62-bit operating system Advantages and Disadvantages of Batch Operating System Boot Block and Bad Block in Operating System Contiguous and Non - Contiguous Memory Allocation in Operating System Control and Distribution Systems in Operations Management Control Program in Operating System Convergent Technologies in Operating System Convoy Effect in Operating System Copy Operating Systems to SSD Core Components of Operating System Core of UNIX Operating System Correct Value to return to the Operating System Corrupted Operating System Cos is Smart Card Operating System Cosmos Operating Systems Examples Generation of Operating System Hardware Solution in Operating System Process Control Block in Operating System Function of Kernel in Operating System Operating System Layers History of Debian Operating Systems Branches and Architecture of Debian Operating Systems Features and Packages of Debian Operating Systems Installation of Operating System on a New PC Organizational Structure and Development in Debian Operating Systems User Interface in Operating System Types Of Memory in OS Operating System in Nokia Multilevel Paging in OS Memory Mapping Techniques in OS Memory Layout of a Process in Operating System Hardware Protection in Operating System Functions of File Management in Operating System Core of Linux Operating System Cache Replacement Policy in Operating System Cache Line and Cache Size in Operating System What is Memory Mapping? Difference Between Network Operating System And Distributed Operating System What is the difference between a Hard link and a Soft Link? Principles of Preemptive Scheduling Process Scheduling Algorithms What is NOS? What is the Interrupt I/O Process? What is Time Sharing OS What is process termination? What is Time-Sharing Operating System What is Batch File File system manipulation What is Message-passing Technique in OS Logical Clock in Distributed System

Priority Scheduling in OS

Priority Scheduling is a type of CPU Scheduling algorithm which is used for process scheduling. In Priority Scheduling, we assign some priorities to each process. The process which has higher priority among all the processes is assigned with the CPU first.

The priority scheduling is of two types:

  1. Preemptive Priority Scheduling.
  2. Non-Preemptive Priority Scheduling.

The priority number, which is assigned to each process, can be different. If the number of priorities which is assigned to each process does not change itself, then such type of priority is known as static priority.

But if the number of priorities is changing itself, then this type of priority is known as Dynamic priority.

Advantages of Priority Scheduling

The advantages of Priority Scheduling are:

  1. Priority scheduling is simple to understand.
  2. Priority scheduling is a user-friendly algorithm.
  3. Based on priority, processes are executed. So, the process which has the highest priority does not need to wait for more time
  4. Priorities in the Priority scheduling are chosen on the basis of time requirements, memory requirements, and user preferences.

Disadvantages of Priority Scheduling

The disadvantages of Priority Scheduling are:

  1. When the multiple processes have the same priorities, then we have to apply another scheduling algorithm.
  2. In priority scheduling, if the system is crashed, then all low-priority processes that are not yet completed will also get lost.
  3. Another disadvantage of Priority Scheduling is starvation. The problem of starvation is a situation that arises when a process does not get the required resources like CPU because another process is holding the resources, and it waits for a long time for the CPU. ‘Aging’ is a technique that is used to remove the problem of starvation. In aging, the system automatically increases the priorities of the processes, waiting for a long time to complete its execution.

Non-Preemptive Priority Scheduling

In this, the processes are scheduled on the priority basis. Each process is assigned a priority. If the process is scheduled, then it will not leave the CPU until it completes the execution.

In Non-Preemptive Priority Scheduling, the process which has the highest priority is scheduled first, and if the processes have the same priority number, then it will be executed according to the First-Come-First-Serve manner. Priorities are assigned based on the resource requirements, time requirements, and Memory requirements.

Example of Non-Preemptive Priority Scheduling

In the following example, we have 7 processes with process ID P1, P2, P3, P4, P5, P6, and P7. The arrival time and burst time of the processes are given in the following table:

Process ID Priority Arrival Time Burst Time Completion time Turnaround time Waiting time Response time
P1 1 0 4 4 4 0 0
P2 5 3 6 23 20 14 17
P3 2 2 5 9 7 2 4
P4 4 5 3 17 12 9 14
P5 6 7 10 33 26 16 23
P6 3 6 5 14 8 3 9
P7 9 8 11 44 36 25 33

The waiting time and turnaround time are calculated with the help of the following formula:

         Waiting Time = Turnaround time – Burst Time

          Turnaround Time = Completion time – Arrival time 

Process waiting time








Average Waiting Time= 0+17+2+9+16+3+25/7



Process Turnaround Time








Average Turnaround Time= 4+20+7+12+26+8+36/7


                                               = 16.14

Priority Scheduling in OS

Preemptive Priority Scheduling

Preemptive Priority Scheduling is a scheduling algorithm that is used when a process enters the ready queue first. We compare the priority of the process with other processes, present in the ready queue and, which the CPU is executing at that point of time. The process which has the highest priority among all other processes will be assigned with the CPU first.

In preemptive priority scheduling, if a new process enters into the ready queue, having a higher priority than the currently running process. In such a case, the CPU is preempted to the newly arrived process, i.e., the current process processing gets stopped, and the new incoming process begins its execution.

Example of Preemptive Priority Scheduling

In the following example, we have 4 processes with process ID P1, P2, P3, and P4. The arrival time and burst time of the processes are given in the following table. (In this example, we assume that higher the number, higher the priority).

Process ID Priority Arrival Time Burst Time Completion Time Turnaround Time Waiting Time
P1 11 0 5 12 12 7
P2 22 1 4 8 7 3
P3 33 2 2 4 2 0
P4 44 4 1 5 1 0

The waiting time and turnaround time are calculated with the help of the following formula.

         Waiting Time = Turnaround time – Burst Time

          Turnaround Time = Completion time – Arrival time 

Priority Scheduling in OS

At time 0, the process P1 arrived with burst time 5 and started its execution. But when the process P2 came with the priority higher than the P1 process. So, process P1 got preempted with P2. P2 began its execution then. This process continued until all the process finished their execution. In preemptive priority scheduling, CPU executes the process only for a single interval of time so that the higher priority process can never skip.

Process Turnaround Time





Average Turnaround Time= 12+7+2+1/4



Process Waiting Time





Average Waiting Time=7+3+0+0/4