Demand paging | Introduction | OS | Lec-22 | Bhanu Priya

Summary

The video explains demand paging, a key aspect of virtual memory management where the CPU requests pages from secondary memory (disk) only when needed. Unlike traditional paging systems, which load all pages in advance, demand paging loads pages on demand, optimizing RAM usage. When a page is not found in main memory, a page fault occurs, transferring control to the operating system to fetch the required page from disk. This efficient swapping technique allows the system to maintain performance by managing limited memory resources effectively.

Takeaways

• 😀 Demand paging is a virtual memory concept where pages are loaded into main memory only when needed.
• 📂 Main memory (RAM) is initially empty when the computer starts, while processes reside in secondary memory (disk).
• 🔄 Pages are transferred from secondary memory to main memory on-demand, rather than preloaded.
• ⚖️ If main memory is full, the system can swap out unused pages to make room for new ones.
• 📥 The CPU requests specific pages for processing; if they are not in main memory, a page fault occurs.
• 🚨 A page fault indicates that the CPU requested a page not currently loaded in RAM.
• 🔄 When a page fault happens, control is transferred from the program to the operating system.
• 💾 The operating system retrieves the requested page from secondary memory and loads it into main memory.
• 🔁 If necessary, the OS may swap out an existing page to accommodate the new one being loaded.
• ⚙️ Demand paging optimizes memory usage by ensuring that only relevant pages are kept in main memory.

Q & A

What is demand paging?

Demand paging is a virtual memory concept where pages are loaded into main memory only when they are needed, rather than in advance.

How does demand paging relate to main memory and secondary memory?

In demand paging, the main memory is where currently active pages reside, while secondary memory (like a disk) holds the complete processes until needed.

What happens when a CPU requests a page that is not in main memory?

If the requested page is not present in main memory, it results in a 'page fault,' prompting the operating system to retrieve the page from secondary memory.

What is the role of the operating system in demand paging?

The operating system manages memory and handles page faults by transferring control from the program to itself, then fetching the required page from secondary memory.

What occurs during a page fault?

During a page fault, the CPU cannot find the requested page in main memory, leading it to request the operating system to bring the page back from secondary memory.

Can you explain the swapping process in demand paging?

Swapping involves moving an inactive page from main memory back to secondary memory to make space for a new page that the CPU demands.

How does main memory get filled in a demand paging system?

Main memory fills as pages are loaded from secondary memory based on demand; if there's no free space, existing pages may be swapped out.

What is the difference between demand paging and traditional paging systems?

While both involve page management, demand paging loads pages only when requested, whereas traditional paging may load pages in advance, using more memory.

What is the significance of frames in main memory?

Frames are the fixed-size blocks in main memory where pages are loaded. Each process is divided into frames, and these are managed to optimize memory usage.

What implications does demand paging have for system performance?

Demand paging can improve system performance by reducing memory usage, but frequent page faults can lead to increased overhead and slower response times.