When the physical memory in the system runs out and a process needs to bring a page into memory then the operating system must decide what to do. It must fairly share the physical pages in the system between the processes running in the system, therefore it may need to remove one or more pages from the system to make room for the new page to be brought into memory. How virtual pages are selected for removal from physical memory affects the efficiency of the system. Linux uses a page aging technique to fairly choose pages which might be removed from the system. This scheme involves every page in the system having an age which changes as the page is accessed.
Old pages are good candidates for swapping.
If the page to be removed came from an image or data file and has not been written to then the page does not need to be saved. Instead it can be discarded and if the process needs that page again it can be brought back into memory from the image or data file again. However, if the page has been written to then the operating system must preserve the contents of that page so that it can be accessed at a later time. This type of page is known as a dirty page and it is saved in a special sort of file called the swap file. These unwanted dirty virtual pages are stored on hard disk in the swap file.