Operating System Questions Long
Demand paging is a technique used in virtual memory management to optimize memory usage by allowing the operating system to load only the necessary pages of a process into physical memory. It is based on the principle of bringing in pages into memory only when they are required, rather than loading the entire process into memory at once.
In demand paging, the virtual memory is divided into fixed-size units called pages, and the physical memory is divided into fixed-size units called frames. The pages of a process are loaded into frames as and when they are needed, based on the demand from the CPU. This allows for efficient utilization of memory resources as only the required pages are loaded, reducing the overall memory footprint.
When a process is initially loaded, only a small portion of it, typically the first few pages, is brought into memory. This is known as the initial page load. As the process executes, it may reference memory locations that are not currently in physical memory. When such a reference occurs, a page fault is generated, indicating that the required page is not present in memory.
Upon a page fault, the operating system performs a series of steps to handle the fault. It first checks if the referenced page is present in secondary storage, such as the hard disk. If the page is present, it is brought into an available frame in physical memory. This process is known as page replacement. If there are no available frames, the operating system selects a victim page to be replaced, using various page replacement algorithms like Least Recently Used (LRU) or First-In-First-Out (FIFO).
Once the required page is brought into memory, the operating system updates the page table of the process to reflect the new mapping between virtual and physical memory. The process is then allowed to continue execution from the point of the page fault.
Demand paging provides several advantages in virtual memory management. It allows for efficient memory utilization by loading only the necessary pages, reducing the amount of physical memory required. It also enables the execution of processes that are larger than the available physical memory, as only the required pages are loaded into memory.
However, demand paging also introduces some overhead. The page faults incurred during the process execution can cause delays as the required pages are brought into memory. Additionally, the page replacement process can introduce additional overhead due to the selection and swapping of pages.
In conclusion, demand paging is a technique used in virtual memory management that brings pages into memory only when they are required. It optimizes memory usage by loading only the necessary pages, allowing for efficient memory utilization and enabling the execution of larger processes. However, it also introduces overhead in the form of page faults and page replacement.