Paging in an operating system eliminates external fragmentation and enables virtual memory, allowing seamless execution even with limited RAM. In this blog, we will explore how Paging works, its benefits, and its challenges in modern operating systems.
In this article, we will explore more about Paging and its importance in operating system.
What is Paging in an Operating System?
Paging is a memory management scheme that permits a process’s non-contiguous physical address space. Paging avoids external fragmentation and the need for compaction. The basic method for implementing paging involves breaking physical memory into fixed-sized blocks called frames and breaking logical memory into blocks of the same size called pages.
When a process is to be executed, its pages are loaded into any available memory frames from their source, a file system, or the backing store. The backing store is divided into fixed-sized blocks that are the same size as the memory frames or clusters of multiple frames.
Let us have a real-world analogy of paging and its operation. Imagine watching Netflix, which is like paging. Let’s see how:
- You don’t download the entire movie at once; instead, it buffers and loads parts of it as needed.
- If you skip forward, it fetches the required section and discards older parts.
- If the internet is slow (like slow disk access), buffering (page faults) happens
That is how modern operating systems efficiently manage memory with paging!
How Paging Works in Memory Management?
Every address generated by the CPU is divided into two parts, including page number or page offset. The page number is used as an index in a page table. The base address is combined with the page offset to define the physical memory address that is sent to the memory unit.
If the size of the logical address space is 2^m and a page size is 2^n bytes, then the high-order m-n bits of a logical address designate the page number, and the n low-order bits designate the page offset.
When a program is loaded into memory, it is divided into fixed-sized blocks called pages. Now these pages don’t have to be stored continuously in RAM, solving the problem of memory fragmentation.
The RAM is also divided into fixed-sized frames (of the same sizes as pages). When a program runs, its pages are loaded into a frame, but not necessarily in order, they can be scattered across RAM.
Since pages are scattered, the OS maintains a Page Table, which acts like a map to track where each page of a process is loaded into the memory. Each and every process has its own page table.
When a program accesses memory, it uses a virtual address like a catalog number in a library. The OS uses the page table to translate this virtual address into a physical address, finding the exact frame where the requested page is stored.
Read more: Process Management in Operating System: Complete Explanation
Handling Missing Pages: Page Faults
Sometimes, the required page is not in memory because not all pages fit in RAM. Now, if the page is not found in the RAM, it is known as a page fault where the CPU misses the page in the memory. In this condition, there are some of the steps that take place internally in the memory manager:
- The operating system pauses the program and fetches the missing page from secondary storage, which is a disk.
- This is called a Page Fault like realizing you need a book that is still in storage.
- The operating system may have to swap out an existing page to make space, using algorithms like FIFO (First In, First Out), LRU (Least Recently Used), and Optimal.
- The FIFO approach removes the oldest page, the LRU approach removes the least used page, and the Optimal approach predicts the future and removes the least needed page.
Advantages of Paging
Paging is a powerful memory management technique that balances efficiency and flexibility. Some of the major advantages of paging include
1. Eliminates External Fragmentation
Since memory is divided into fixed-sized frames, there are no gaps like in contiguous memory allocation, ensuring better memory utilization. Paging solves this problem by breaking both physical memory and processes into fixed-sized blocks (pages and frames). This means that a process doesn’t need a continuous block, reducing wastage and improving memory utilization.
2. Efficient Memory Management
Programs don’t need to be stored continuously in RAM, making it easier to load multiple processes and support multitasking. Since processes are stored in non-contiguous memory locations, available memory is used more efficiently. Multiple small processes can coexist in RAM without waiting for a single large free block to be available.
Read more: Understanding OS Fundamentals: Beginners Overview
3. Faster Process Execution
The operating system can load the necessary pages of a process into memory, reducing the time required to start and run applications. A process does not need to be fully loaded into RAM before execution. The operating system can load only the required pages initially and bring in other pages on demand. This is particularly useful for large applications and multitasking environments, where multiple processes share the CPU efficiently.
4. Supports Virtual Memory
Not all pages of a process need to be in RAM at once; some can be kept on the disk and fetched only when needed, allowing processes to run even with limited RAM. Modern systems use virtual memory, where processes can run even if they don’t fit entirely in RAM. Unused pages remain in secondary storage and are fetched only when required; this particular process is called demand paging. This allows systems to run applications that are larger than the available RAM, enhancing system compatibility.
Read more: Functions of an Operating System: Complete Explanation
5. Easy Processes Swapping (Swapping In/Out)
Since each page is independent, processes can be swapped in and out efficiently, improving the system’s performance. Since processes are divided into pages, the OS can easily swap pages in and out of memory without needing to reload the entire process. This helps in multitasking, allowing the CPU to switch between processes efficiently.
6. Better Security and Isolation
Each process has its own page table, preventing accidental or malicious access to another process’s memory. Each process has its own page table, preventing one process from accessing another process’s memory. This enhances security, as unauthorized access or memory corruption is avoided.
7. Simplifies Memory Allocation
The operating system doesn’t need to find a large continuous space for a process—just enough free frames, making allocation simpler and faster. The OS does not need to find large contiguous memory spaces—it just needs free frames anywhere in RAM.
Disadvantages of Paging In Operating System
Paging is a fundamental memory management technique used in modern operating systems. It offers several benefits in terms of memory efficiency, process execution, and multitasking, but it also comes with some drawbacks like overhead and performance issues. Some of the major disadvantages of paging are mentioned below:
1. Increases Memory Overhead
The page table itself consumes memory, especially for large processes, as it needs to store mappings for every page.
2. Page Table Access Overhead
Every memory access requires two lookups, including one to fetch the page table entry and another to access the actual memory location. This can slow down execution, though TLBs (Translation Lookaside Buffers) help reduce this issue.
3. Page Faults Slow Down Performance
If a required is not in the memory, a page fault occurs, causing a delay as the operating system fetches the missing page from the hard disk which is much slower than RAM.
4. Internal Fragmentation
If a process does not fully use its allocated pages, the remaining space in that page is wasted (for example, if a page is 4 KB but the process needs 3 KB, the extra 1 KB is wasted)
Read more: Important Topics in Operating System: Career Wise Explanation
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Paging In Operating System FAQs
Q1. What is paging in an operating system?
Ans. Paging is a memory management scheme that permits a process's non-contiguous physical address space. Paging avoids external fragmentation and the need for compaction.
Q2. What is Page Fault?
Ans. Sometimes, the required page is not in memory because not all pages fit in RAM. Now, if the page is not found in the RAM, it is known as a page fault where the CPU misses the page in the memory.
Q3. What are the advantages of paging in OS?
Ans. Paging is a fundamental memory management technique used in modern operating systems. It offers several benefits in terms of memory efficiency, process execution, and multitasking.
Q4. What are the disadvantages of paging in OS?
Ans. Paging offers several benefits but it also comes with a few disadvantages, including internal fragmentation, increased memory overhead, page table access overhead, and many more.
