Skip to main content

Demand Paging In Operating System

   
   ⇰   DEMAND PAGING :-
  A demand paging system is basically advanced version of  paging system with swapping where processes reside in secondary memory and pages are loaded into memory only on demand, not in advance.  When we want to execute a process, we swap it into memory. Rather than swapping the entire process into memory.

   Let us consider an executable program loaded from disk into memory. One option is to load the entire program in physical memory at program execution time. It may be possible that we may not initially need the entire program in memory. Suppose a program starts with a list of available options from which the user is to select. Loading the entire program into memory results in loading the executable code for all options, regardless of whether or not an option is ultimately selected by the user. An alternative strategy is to load pages only as they are needed. This technique is known as demand paging. 

   Demand paging is commonly used in virtual memory system. With demand paging, virtual memory pages are loaded only when they are demanded during program  execution. Pages that are never accessed are thus never loaded into physical memory. We use a lazy swapper for this. A lazy swapper never swaps a page into memory unless that page will be needed. 


  According to the concept of Virtual Memory, when some process is executing, only a part of the process needs to be present in the main memory which means that only a few pages will only be present in the main memory at any time. However, to decide which pages are need to kept in the main memory and which need to be kept in the secondary memory is difficult because we cannot say in advance that a process will require a particular page at particular time. Therefore, to overcome this problem, there is a concept called demand paging is introduced. 

 While executing a program, if program reference a page which is not available in the main memory because it was swapped out a few times 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.


  ⇰  ADVANTAGES :-
   i> Demand paging provides large virtual memory. 
  
  ii> There is more efficient use of memory after applying this. 

 iii> It provides unconstrained multiprogramming. 

 iv> There is no limit on degree of multiprogramming. 


Share, Follow and please comment if you find anything incorrect or to share more information about the topic discussed above
Labels:

Comments

Post a Comment

Please comment.

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 shar...
8 comments
Read more

Process & Its state And process control block :-

                ⇰  PROCESS :- A process can be thought of as a program in execution. Means when any program is executed it becomes process. A processwill need certain resources such as CPU time , memory, files and I/O devices to complete its task. These resources are allocated to the process either when it is created or at the time of execution.             A process is the unit of work in most systems. A system consistes of a collection of processes. All these processes may execute concurrently. Traditionally a process contained only a single thread. Most modern operating ststems now supports processes that have multiple threads.         The operating system is responsible for several important works of process management as - the creation and deletion of process, the schrduling of process, communication and deadlock handling of process. Process is broudly divided into two ...
Post a Comment
Read more

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...
3 comments
Read more