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 Demand Paging?

Demand Paging is defined as a process in which the pages are loaded into the memory (when the page fault occurs) or on-demand.

It consists of the following steps:

What is Demand Paging
  1. If there is a page that CPU wants to refer, but currently in the Main Memory, that page is not available, then in this situation, an interrupt is generated by the CPU, which indicates that there is memory access fault.
  2. With the help of an operating system, the process which is interrupted is put into the blocking state. To proceed the execution, the operating system takes the page which is needed into the Main Memory.
  3. For the required page, the operating system will search in the logical address space.
  4. From the logical address space to the physical address space, the needed page will be carried.

We use page replacement algorithms to decide which page is replaced in the physical address space. Accordingly, we update the page table.

  • Then, to continue the execution of the program, the signal is sent to the CPU, and the CPU will again place the process into the ready state.

Therefore, the operating system uses the above steps at the time when page fault has occurred, and the needed page is taken into the memory.

Advantages of Demand Paging

The advantages of demand paging are:

  1. Memory can be used more efficiently.
  2. If we use demand paging, then we can have a large virtual memory.
  3. By using demand paging, we can run programs that are larger than physical memory.
  4. In demand paging, there is no requirement of compaction.
  5. In demand paging, the sharing of pages is easy.
  6. In demand paging, partition management is simple because of the fixed partition size and the discontinuous loading.

Disadvantages of Demand Paging

The disadvantages of demand paging are:

  1. In demand paging, there may be a chance of internal fragmentation.
  2. The memory access time is longer (page table lookup).

What is a Page Fault?

Page Fault is a condition in which the page that is referred or requested does not exist in the main memory.  A Page fault is also known as Page miss.

In other words, a page fault is defined as a condition in which the OS (operating system) cannot find the needed data or information in the virtual memory.


The process of swapping means removing all the pages of the process from memory, or marking the pages so that we can remove the pages with the help of the page replacement process.

If the process is suspended, it means the process cannot run. but we can swap out the process for sometime. After some time, the process can be swapped back by the system from the secondary memory to the primary memory. The situation where a process is busy with the pages being swapped in and out of that situation is known as thrashing.



In thrashing, for any process, there are only some pages which are present in the main memory so we can maintain more processes in the memory. With this, the time is also saved because, for unused pages, there is no requirement of swap-in and swap-out of memory.

Although the operating system should be crafty so that it can handle this scheme. Practically in the steady-state, pages of the processes will occupy the entire main memory. So, the operating system and the processor have direct access to more processes. Therefore, it is must when the operating system carries one page in, and then another page should be thrown out. When the operating system throws a page out before the usage of the page, then in this condition, the operating system will again get the page instantly.

Instead of executing an instruction, system takes more time in the swapping of the pages. So, there is a need of a suitable page replacement algorithm.

We can see in the below diagram that the initial degree of multiprogramming is up to some extent of point (lamda), the utilization of the CPU is too high and 100% the resources of the system are used but if in case the degree of multiprogramming is increased then the utilization of the CPU will fall down extremely, and for the page replacement the system will spent its more time and time taken to finish the process execution is also increased. And this condition is known as thrashing.

Causes of Thrashing

The causes of thrashing are:

  1. Lack of Frames
  2. The high degree of multiprogramming
  1. Lack of Frames: - In this, if the process has less frames, then there will be less pages of that process that are able to exist in the memory. So, there is a need of frequent swapping in and frequent swapping out, and that may cause thrashing. So, in order to avoid thrashing, we need a suitable number of frames for allocation each process.
  2. High Degree of Multiprogramming: - In the memory, if the processes are getting increased, then the number of frames allocated to each process will decrease. So, in this way, there will be a fewer frame available for every process. And because of that, there will be more page fault, and the time of CPU get wasted only in the swapping in and out of pages, and the utilization of the CPU will also be decreased.

Recovery of Thrashing

  1. If there is already a thrashing in the system, then the mid-term scheduler will be instructed that it suspend a few of the processes to recover the system from the thrashing.
  2. To improve the thrashing, we do not permit the system to enter into a thrashing by instructing the long-term scheduler that, after the threshold, it will not take any process into the memory.