The system can swap out those pages and free the memory for other applications or even for the disk cache. Second, a significant number of the pages used by an application during its startup phase may only be used for initialization and then never used again. First, when the system requires more memory than is physically available, the kernel swaps out less used pages and gives memory to the current application (process) that needs the memory immediately. Swapping is necessary for two important reasons. The combined sizes of the physical memory and the swap space is the amount of virtual memory available.
Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory. Linux divides its physical RAM (random access memory) into chucks of memory called pages.
