What is the purpose of swapping in virtual memory?


It will help a lot to read about how virtual memory works before going forward.

Swapping is exchanging data between the hard disk and the RAM

The goal of the virtual memory technique is to make an application think that it has more memory than actually exists. If you read the recommended question then you know that the virtual memory manager (VMM) creates a file on the hard disk called a swap file. Basically, the swap file (also known as a paging file) allows the application to store any extra data that can’t be stored in the RAM – because the RAM has limited memory. Keep in mind that an application program can only use the data when it’s actually in the RAM. Data can be stored in the paging file on the hard disk, but it is not usable until that data is brought into the RAM. Together, the data being stored on the hard disk combined with the data being stored in the RAM comprise the entire data set needed by the application program.

So, the way virtual memory works is that whenever a piece of data needed by an application program cannot be found in the RAM, then the program knows that the data must be in the paging file on the hard disk.

But in order for the program to be able to access that data, it must transfer that data from the hard disk into the RAM. This also means that a piece of existing data in the RAM must be moved to the hard disk in order to make room for the data that it wants to bring in from the hard disk. So, you can think of this process as a trade in which an old piece of data is moved from the RAM to the hard disk in exchange for a ‘new’ piece of data to bring into the RAM from the hard disk. This trade is known as swapping or paging. Another term used for this is a ‘page fault’ – which occurs when an application program tries to access a piece of data that is not currently in the RAM, but is in the paging file on the hard disk. Remember that page faults are not desirable since they cause expensive accesses to the hard disk. Expensive in this context means that accessing the hard disk is slow and takes time.

The Purpose Of Swapping

So, we can say that the purpose of swapping, or paging, is to access data being stored in hard disk and to bring it into the RAM so that it can be used by the application program. Remember that swapping is only necessary when that data is not already in the RAM.

Excessive Swapping Causes Thrashing

Excessive use of swapping is called thrashing and is undesirable because it lowers overall system performance, mainly because hard drives are far slower than RAM.

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