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What is Demand Paging in Operating System?

Last Updated : 12 Mar, 2024
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The concept of query navigation in the operating system. This concept says that we should not load any pages into the main memory until we need them, or keep all pages in secondary memory until we need them.

What is Demand Paging? 

Demand paging can be described as a memory management technique that is used in operating systems to improve memory usage and system performance. Demand paging is a technique used in virtual memory systems where pages enter main memory only when requested or needed by the CPU.

In demand paging, the operating system loads only the necessary pages of a program into memory at runtime, instead of loading the entire program into memory at the start. A page fault occurred when the program needed to access a page that is not currently in memory. The operating system then loads the required pages from the disk into memory and updates the page tables accordingly. This process is transparent to the running program and it continues to run as if the page had always been in memory.

What is Page Fault?

The term “page miss” or “page fault” refers to a situation where a referenced page is not found in the main memory.

When a program tries to access a page, or fixed-size block of memory, that isn’t currently loaded in physical memory (RAM), an exception known as a page fault happens. Before enabling the program to access a page that is required, the operating system must bring it into memory from secondary storage (such a hard drive) in order to handle a page fault.

In modern operating systems, page faults are a common component of virtual memory management. By enabling programs to operate with more data than can fit in physical memory at once, they enable the efficient use of physical memory. The operating system is responsible for coordinating the transfer of data between physical memory and secondary storage as needed.

What is Thrashing?

Thrashing is the term used to describe a state in which excessive paging activity takes place in computer systems, especially in operating systems that use virtual memory, severely impairing system performance. Thrashing occurs when a system’s high memory demand and low physical memory capacity cause it to spend a large amount of time rotating pages between main memory (RAM) and secondary storage, which is typically a hard disc.

It is caused due to insufficient physical memory, overloading and poor memory management. The operating system may use a variety of techniques to lessen thrashing, including lowering the number of running processes, adjusting paging parameters, and improving memory allocation algorithms. Increasing the system’s physical memory (RAM) capacity can also lessen thrashing by lowering the frequency of page swaps between RAM and the disc.

Pure Demand Paging 

Pure demand paging is a specific implementation of demand paging. The operating system only loads pages into memory when the program needs them. In on-demand paging only, no pages are initially loaded into memory when the program starts, and all pages are initially marked as being on disk.

Operating systems that use pure demand paging as a memory management strategy do so without preloading any pages into physical memory prior to the commencement of a task. Demand paging loads a process’s whole address space into memory one step at a time, bringing just the parts of the process that are actively being used into memory from disc as needed.

It is useful for executing huge programs that might not fit totally in memory or for computers with limited physical memory. If the program accesses a lot of pages that are not in memory right now, it could also result in a rise in page faults and possible performance overhead. Operating systems frequently use caching techniques and improve page replacement algorithms to lessen the negative effects of page faults on system performance as a whole.

Working Process of Demand Paging

So, let us understand this with the help of an example. Suppose we want to run a process P which have four pages P0, P1, P2, and P3. Currently, in the page table, we have pages P1 and P3.

Demand Paging

Therefore, the operating system‘s demand paging mechanism follows a few steps in its operation.

  • Program Execution: Upon launching a program, the operating system allocates a certain amount of memory to the program and establishes a process for it.
  • Creating page tables: To keep track of which program pages are currently in memory and which are on disk, the operating system makes page tables for each process.
  • Handling Page Fault: When a program tries to access a page that isn’t in memory at the moment, a page fault happens. In order to determine whether the necessary page is on disk, the operating system pauses the application and consults the page tables.
  • Page Fetch: The operating system loads the necessary page into memory by retrieving it from the disk if it is there.
  • The page’s new location in memory is then reflected in the page table.
  • Resuming the program: The operating system picks up where it left off when the necessary pages are loaded into memory.
  • Page replacement: If there is not enough free memory to hold all the pages a program needs, the operating system may need to replace one or more pages currently in memory with pages currently in memory. on the disk. The page replacement algorithm used by the operating system determines which pages are selected for replacement.
  • Page cleanup: When a process terminates, the operating system frees the memory allocated to the process and cleans up the corresponding entries in the page tables.

Advantages of Demand Paging 

So in the Demand Paging technique, there are some benefits that provide efficiency of the operating system.

  • Efficient use of physical memory: Query paging allows for more efficient use because only the necessary pages are loaded into memory at any given time.
  • Support for larger programs: Programs can be larger than the physical memory available on the system because only the necessary pages will be loaded into memory.
  • Faster program start: Because only part of a program is initially loaded into memory, programs can start faster than if the entire program were loaded at once.
  • Reduce memory usage: Query paging can help reduce the amount of memory a program needs, which can improve system performance by reducing the amount of disk I/O required.

Disadvantages of Demand Paging 

  • Page Fault Overload: The process of swapping pages between memory and disk can cause a performance overhead, especially if the program frequently accesses pages that are not currently in memory.
  • Degraded performance: If a program frequently accesses pages that are not currently in memory, the system spends a lot of time swapping out pages, which degrades performance.
  • Fragmentation: Query paging can cause physical memory fragmentation, degrading system performance over time.
  • Complexity: Implementing query paging in an operating system can be complex, requiring complex algorithms and data structures to manage page tables and swap space.

FAQs on Demand Paging

Q.1: How does demand paging differ from swapping?


Demand paging is a method for loading only the necessary parts of a program into RAM as needed, while swapping involves moving entire processes in and out of RAM. Demand paging is more granular and efficient.

Q.2: What happens during a page fault?


When a page fault occurs, the operating system:

  • Identifies the missing page.
  • Retrieves it from secondary storage.
  • Updates the page table to reflect the new location in RAM.
  • Resumes the interrupted process.

Q.3: What are some strategies to optimize demand paging?


  • Use efficient page replacement algorithms.
  • Adjust the size of the page file or swap space.
  • Ensure sufficient physical memory to reduce page faults.
  • Profile and optimize applications to minimize memory usage.

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