Lec28 Page replacement Introduction| FIFO page replacement algorithm with example| Operating System

Summary

This video explains memory management concepts in operating systems, focusing on virtual memory, demand paging, and page replacement. It highlights how processes are divided into pages, with only necessary parts loaded into main memory, allowing multiple processes to run concurrently despite memory limitations. The script discusses the occurrence of page faults and how the operating system handles them by swapping pages between main memory and secondary memory. It also covers the First-In-First-Out (FIFO) page replacement algorithm, providing a practical example of how it works and how page faults and hit ratios are calculated.

Takeaways

• 😀 Processes must be loaded into main memory for execution; they cannot be executed directly from secondary memory.
• 😀 The size of main memory is finite, which can limit how many processes can run simultaneously.
• 😀 When a process is too large for main memory, only parts (pages) of the process may be loaded to optimize space usage.
• 😀 Virtual memory allows multiple processes to reside in main memory by using paging, making it seem like the memory size is larger than it really is.
• 😀 Demand paging is a technique where only the required pages of a process are loaded into main memory when needed.
• 😀 A page fault occurs when a requested page is not in main memory, requiring the operating system to load it from secondary memory.
• 😀 If the main memory is full, the operating system uses page replacement algorithms to decide which pages to swap out and bring in new ones.
• 😀 The page replacement algorithm aims to minimize page faults, and the FIFO (First In, First Out) is one such method.
• 😀 The operating system maintains a page table that keeps track of which pages of which processes are currently in main memory.
• 😀 The FIFO page replacement algorithm selects the oldest page in memory to be swapped out when a new page needs to be loaded.
• 😀 The FIFO algorithm involves tracking page references and making decisions based on the order in which pages were loaded into memory.

Q & A

What is the main requirement for executing a process?

A process must be loaded into the main memory (physical memory) to be executed. If it is not in memory, it cannot be executed.

What happens if the size of a process exceeds the available size of main memory?

If a process is larger than the main memory, it cannot be fully loaded at once. Only portions of the process, called pages, are loaded into memory as needed.

What is the advantage of loading only parts of a process into main memory?

By loading only the necessary pages of a process, more processes can be kept in memory at the same time, which optimizes memory usage.

What is the concept of 'virtual memory'?

Virtual memory gives the illusion of a larger main memory by allowing the operating system to load only parts of processes into physical memory and use secondary memory (e.g., hard disk) for the rest.

What is 'demand paging'?

Demand paging is when only the required pages of a process are loaded into memory on demand, instead of loading the entire process into memory upfront.

What happens when a page is not available in memory when needed by the CPU?

If a page is not in memory, a page fault occurs. The operating system will locate the page in secondary memory and load it into the main memory.

What is a page replacement algorithm, and why is it needed?

A page replacement algorithm decides which pages to swap out of memory when space is needed for new pages. It is essential when the main memory is full and more pages are required.

What is the FIFO (First-In-First-Out) page replacement algorithm?

The FIFO algorithm replaces the oldest page in memory when new pages need to be loaded, based on the order in which pages were loaded into memory.

How does the operating system track which pages are in memory and which are in secondary memory?

The operating system uses a page table to keep track of where each page is located. Each entry in the table has a valid or invalid bit to indicate whether the page is in main memory or secondary memory.

What is the 'hit ratio' in page replacement algorithms?

The hit ratio is the number of times a requested page is already in memory (a page hit) divided by the total number of page requests.

In the FIFO page replacement example, how are page faults and hits calculated?

Page faults occur when a requested page is not in memory. The hit ratio is calculated by dividing the number of page hits by the total number of page requests. In the example, there were 6 page hits and 13 page faults out of 19 total requests.