Virtual Memory in Operating System

This is an PPT of Operating System. It include the following topic "Virtual Memory in Operating System".

1. BIRLA INSTITUTE OF TECHNOLOGY MESRA, JAIPUR CAMPUS NAME :- Nikhil Agrawal ROLL NO :- MCA/25004/18 TOPIC:- Virtual Memory

2. Introduction Virtual Memory Basics Demand Paging Page Replacement Policies

3. Virtual Memory Basics Virtual memory  is a  memory  management capability that uses hardware and software to allow a computer to compensate for physical memory  shortages by temporarily transferring data from random access  memory ( RAM ) to disk storage. The main visible advantage of this scheme is that programs can be larger than physical memory. Virtual memory serves two purposes. First, it allows us to extend the use of physical memory by using disk. Second, it allows us to have memory protection, because each virtual address is translated to a physical address.

4. In modern operating systems, before a program is loaded into main memory, it is divided into chunks, called PAGES. Each PAGE is loaded into memory locations called PAGE FRAMES. If the PAGES are the exact same size as the PAGE FRAMES (and the same size as the disk sectors), this scheme works very well. The empty page frame does not have to be contagious.

5. Memory Manager The Memory Manager prepares a program for execution by doing the following: Determine the number of pages in the program Locate enough empty page frames in main memory Load all the program’s pages into them The program is stored in the secondary memory. The memory management unit (MMU) transfers the currently needed part of the program from the secondary memory to the main memory for execution. This to and from movement of instructions and data (parts of a program) between the main memory and the secondary memory is called Swapping.

6. Memory management

7. Demand paging A demand paging system is quite similar to a paging system with swapping where processes reside in secondary memory and pages are loaded only on demand, not in advance.  When a context switch occurs, the operating system does not copy any of the old program’s pages out to the disk or any of the new program’s pages into the main memory Instead, it just begins executing the new program after loading the first page and fetches that program’s pages as they are referenced.

8. Demand paging Swapping While executing a program, if the program references a page which is not available in the main memory because it was swapped out a little ago, the processor treats this invalid memory reference as a  page fault  and transfers control from the program to the operating system to demand the page back into the memory.

9. Demand paging Advantages Large virtual memory. More efficient use of memory. There is no limit on degree of multiprogramming. Disadvantages Number of tables and the amount of processor overhead for handling page interrupts are greater than in the case of the simple paged management techniques.

10. Page Replacement Algorithms In a computer operating system that uses paging for virtual memory management, page replacement algorithms decide which memory pages to page out (swap out, write to disk) when a page of memory needs to be allocated. Paging happens when a page fault occurs and a free page cannot be used to satisfy the allocation, either because there are none, or because the number of free pages is lower than some threshold.

11. First - In - First - Out (FIFO) First-in-first-out is very easy to implement. The FIFO algorithm selects the page for replacement that has been in memory the longest time. The page to be removed is easily determined because its identification number is at the top of the FIFO stack. The FIFO replacement policy has the advantage of being easy to implement. It has the disadvantage that under certain circumstances pages are removed and loaded from memory too frequently.

12. FIFO Algorithm Consider a paging system having capacity of 3 pages. The execution of a program requires references to five distinct pages P1 , P2 , P3 , P4 and P5 . The pages are executed in the following sequence: P2 P3 P2 P1 P5 P2 P4 P5 P3 P2 P5 P2

13. Least Recently Used (LRU) The least recently used page (LRU) replacement algorithm keeps track of page usage over a short period of time. The LRU algorithm can be implemented by associating a counter with every page that is in main memory. When a page is referenced, its associated counter is set to 0. At fixed intervals of time, the counters associated with all pages presently in memory are incremented by 1. The least recently used page is the page with the highest count. The counters are often called aging registers, as their count indicates their age, that is, how long their associated pages have been referenced.

14. LRU Algorithm Consider a paging system having capacity of 3 pages. The execution of a program requires references to five distinct pages P1 , P2 , P3 , P4 and P5 . The pages are executed in the following sequence: P2 P3 P2 P1 P5 P2 P4 P5 P3 P2 P5 P2

15. Optimal (OPT) The optimal policy selects that page for replacement for which the time to the next reference is longest. This algorithm results in fewest number of page faults. But, this algorithm is impossible to implement. At the time of page fault , the operating system has no way of knowing when each of the pages will be referenced next. However , it does serve as a standard against which to judge other algorithms.

16. Optimal (OPT) Consider a paging system having capacity of 3 pages. The execution of a program requires references to five distinct pages P1 , P2 , P3 , P4 and P5 . The pages are executed in the following sequence: P2 P3 P2 P1 P5 P2 P4 P5 P3 P2 P5 P2

17. THANK YOU.