Skip to main content

Page Replacement In Operating System

As we know if we have to increase our degree of multiprogramming, we will over-allocate memory. For this we use demand paging technique.But if CPU demands a page which is not present in memory then page fault occurs. In this situation the command of CPU is taken by O/S and O/S replace the page with the demanded page. Then return command to the CPU then CPU will perform its execution on that demanded page.


Therefore, we can say page replacement occur when CPU demands any page which is not in memory and the memory is full, then one page is to replace from memory by demanded page. And page fault happens when a running program accesses a memory page that is mapped into the virtual address space, but not loaded in physical memory.


Page replacement takes the following approach. If no frame is free, we find one that is not currently being used and freee it. We can free a frame by writing its contents to swap space and changing the page table to indicate that the page is no longer in memory. We modify the page-fault service routine to include page replacement by following method :-

1. Find the location of the desired page on the disk.

2. Find a free frame :-

   a. If there is a free frame, use a page-replacement algorithm to select a victim frame.

   b. If there is no free frame, use a page-replacement algorithm to select a victim frame.

   c. Write the victim frame to the disk, change the page and frame tables accordingly.

3. Read the desired page into the newly freed frame, change the page and frame tables.

4. Continue the user process from where the page fault occured.


If no frames are free, two page transfers are required, one for out and one for in. This situatioon effectively doubles the page-fault service time and increase the affective access time accordingly. We must solve two major problems to implement demend paging :-

We must develop a frame-allocation algorithm and a page-replacement algorithm. That is, if we have multiple processes in memory, we must decide how frames to allocate to each process, and when page replacement is required, we must select the flames that are to be replaced. Designing appropriate algorithms to solve these problems is an important task, because disk I/O is so expensive. 


The page replacement algorithm decides which memory page is to be replaced. The process of replacement is sometimes called swap out or write to disk. Page replacement is done when the requested page is not found in the main memory. For replacement of page there are many different page-replacemrnt algorithms are present.

Every O/S  probably has its own page-replacement algorithms. We select a particular page-replacement algorithm for O/S which has lowest page-fault rate. Obviously, as the number of frames available increases, the number of page-fault decreases. We will watch various types of page-replacement algorithms in next post.
Share, Follow and please comment if you find anything incorrect or to share more information about the topic discussed above.

Comments

Popular posts from this blog

Process Scheduling And Types of Process Schedular :-

        ⇰ PROCESS SCHEDULING Process Scheduling  is a task  of Operating System that schedules processes of different states like new, ready, waiting, terminated  and running.This scheduling helps in allocation of CPU time for each process, and Operating System allocates the CPU time for each procss. And the process scheduling plays important role to keep the CPU busy all the time.  ⏩   Followings are some objectives of Process Scheduling :-  i > To increase the amount of users within acceptable response times.  ii > To maintain the balance between response and utilization of system. iii > To decrease the enforce priorities and  give reference to the processes holding the key resources.      ⇰  PROCESS SCHEDULAR A scheduler carries out the pro cess scheduling work. Schedulers are often implemented so they keep all computer resources busy and  allows multiple users to share system resources  to achieve  multiprogramming .  There are  mainy three types of pro

Tokens and its types in 'C'

   Tokens are the smallest individual unit of a program or in simple words it is a main part of C program.Tokens are the building blocks of any program. The smallest individual and basic unit of a C programming is called c tokens.      *    Normally there are six types of tokens in C:- i> Keywords:-          Keywords are special words that are used to give a special meaning to the program and can't be used as variable and constant.They are basically a sequence of characters that have fixed to mean. For example:-                 auto     double      long     break                 float    short        char     if                while    continue   int       void etc. All keywords must be written in lower case.It is 32 in number. ii> Identifiers:-         The identifiers are basically a token. It can be a variable's name or a label's name. So identifiers are actually a user defined data. But there are certain rules to frame an identifier. They are as follo

Micromax 'The Indian Smart Phone Company'

Micromax  is an Indian  consumer electronics  company headquartered in  Gurgaon ,  Haryana . It was established as an  IT   Software  company operating in the  Embedded Devices   Domain . It later entered the  Mobile   Handset  business. Micromax was incorporated as Micromax Informatics Ltd. on 29 March 2000 by Zeeshan Ali Zaidi. It began selling mobile telephones in 2008,  focusing on  low pricing to compete with international brands. By 2010, Micromax was one of the largest domestic companies making handsets in the low-cost feature phone segment in India. As of Q3 2014, Micromax is the  Tenth Largest   Smart phone  vendor in the world. The company is facing stiff competition from Chinese companies that are penetrating the Indian market. The company also owns  YU Televentures , which sells its products under the brand name YU. The company has also introduced handsets with innovative features. For instance, Micromax's co-founder  Rahul Sharma  once saw a  public call