Demand Paging in OS (Operating System): Example, Advantages, and Working!!

Demand Paging in OS Demand paging is a process of swapping in the Virtual Memory system. In this process, all data is not moved from hard drive to main memory because while using this demand paging, when some programs are getting demand then data will be transferred.

Why Demand Paging?

Each process has a multiple pages. But, it not enough to insert all pages of that process into primary memory because, RAM size is limited as well. Therefore, while getting to execute all process then loading the pages as per the requirement.

It may be probability that any application does not require all its pages for executing the applications.

Demand Paging Working Demand paging system is totally depend on the page table implementation because page table helps to maps logical memory to physical memory. Bitwise operators are implemented in the page table to indication that pages are ok or not (valid or invalid).

All valid pages are existed into primary memory, and other side invalid pages are existed into secondary memory.

Valid-Invalid Bit • Along with every page table entry, a valid or invalid bit is linked (whereas 1 represents existed in the memory and 0 denotes not presented in the memory).

• Firstly, a valid and invalid bit is must be set to 0 for all table entries that deals to ‘Page Fault’.

• When the bit is going to set to ‘Valid’ then attached page is both legal and in the memory.

• If bit is ‘Invalid’, then it denotes that page is either not valid or page is valid but at present time not available on the disk.

• For those pages, which are fetched into memory, then page table is set as usual.

• But those pages, which are not available in the main memory, and then page table is either simply named as invalid or it has the address of page on the disk.

Demand Paging Example Memory Access Time = 200 nanoseconds Average Page Fault Service Time = 8 milliseconds EAT = (1-p)*200+p(8 milliseconds) = (1-p)*200+p*8000000 = 200+p*7999800 EAT means direct proportional to the page fault in OS rate.

Demand Paging Advantages There are many benefits of demand paging in OS such as: •

Memory can be utilized with better efficiently.

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We have to right for scaling of virtual memory.

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If, any program is larger to physical memory then It helps to run this program.

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No need of compaction.

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Easy to share all pages

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Partition management is more simply.

Demand Paging Disadvantages • It has more probability of internal fragmentation.

• Its memory access time is longer.

• Page Table Length Register (PTLR) has limit for virtual memory • Page map table is needed additional memory and registers.

Pure Demand Paging In the initially stage, if anyone page is not loaded into primary memory then page faults are occurred, and then paged are loaded with on demanding of the process. This process is known as “Pure Demand Paging”.

Pure demand paging is not able to load single page into main memory, otherwise it will be fired the PageFault.

In pure demand paging, when process will be started up then all memory is swapped from auxiliary memory to primary memory.

Demand Segmentation in OS Operating system implements demand segmentation like demand paging. While using “Demand Paging”, if it to get lack of hardware resources, then OS implements the demand segmentation.

Segment table keeps all information related to demand segmentation such as valid bit because on the behalf of valid bit can be specified that segment has existed in the physical memory or not.

If, anytime physical memory is not capable to store their segments then to get segment fault, and then it try to fetch required segments from physical memory. Similar to page fault.

Demand segmentation helps to decrease the number of page faults in the paging system.

Demand Paging in OS Related Things What is Page Fault?

Page fault in OS is getting to happen whenever the page that is requested doesn’t present in the primary memory. Page fault is also called the ‘Page Miss’.

What is Swapping?

Swapping process comprises either deleting all pages of the process from the main memory or creating the pages so that we are eligible to remove the pages through the page replacement techniques.

What is Thrashing?

In thrashing, for any process, there are only few pages, which are available in the primary memory, so we are able to maintain many processes in the main memory.

The time is also kept save because for waited pages (Unused), there is not necessary of swap-in and swap-out of memory.

Frequently Asked Questions What is demand paging in OS explain with example?

Demand paging is a process where data is transferred from secondary memory to main memory as per the demand that means all data is not saved into primary memory (RAM) because RAM has limited space for execution.

What is demand paging in virtual memory?

This concept already we have explain above in this article, you can read them.

Frequently Asked Questions What is demand paging how it is implemented?

Virtual memory is implemented by using Demand Paging in operating system, and then you can read this article: Virtual Memory in OS – Complete Guide!!

What is performance of demand paging in OS? Demand paging can affect the performance of a computer system, so you should be calculate the effective access time for a demand paged.

Effective access time = (1 – p)*ma + p*page fault time

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