Virtual Memory Optimization Guide

Virtual Memory

Back in the 'good old days' of command prompts and 1.2MB floppy disks, programs needed very little RAM to run because the main (and almost universal) operating system was Microsoft DOS and its memory footprint was small. That was truly fortunate because RAM at that time was horrendously expensive. Although it may seem ludicrous, 4MB of RAM was considered then to be an incredible amount of memory.

However when Windows became more and more popular, 4MB was just not enough. Due to its GUI (Graphical User Interface), it had a larger memory footprint than DOS. Thus, more RAM was needed.

Unfortunately, RAM prices did not decrease as fast as RAM requirement had increased. This meant that Windows users had to either fork out a fortune for more RAM or run only simple programs. Neither were attractive options. An alternative method was needed to alleviate this problem.

The solution they came up with was to use some space on the hard disk as extra RAM. Although the hard disk is much slower than RAM, it is also much cheaper and users always have a lot more hard disk space than RAM. So, Windows was designed to create this pseudo-RAM or in Microsoft's terms - Virtual Memory, to make up for the shortfall in RAM when running memory-intensive programs.

How Does It Work?

Virtual memory is created using a special file called a swapfile or paging file.

Whenever the operating system has enough memory, it doesn't usually use virtual memory. But if it runs out of memory, the operating system will page out the least recently used data in the memory to the swapfile in the hard disk. This frees up some memory for your applications. The operating system will continuously do this as more and more data is loaded into the RAM.

However, when any data stored in the swapfile is needed, it is swapped with the least recently used data in the memory. This allows the swapfile to behave like RAM although programs cannot run directly off it. You will also note that because the operating system cannot directly run programs off the swapfile, some programs may not run even with a large swapfile if you have too little RAM.


Swapfile Vs. Paging File

We have all been using the terms swapfile and paging file interchangeably. Even Microsoft invariably refers to the paging file as the swapfile and vice versa. However, the swapfile and paging file are two different entities. Although both are used to create virtual memory, there are subtle differences between the two.

The main difference lies in their names. Swapfiles operate by swapping entire processes from system memory into the swapfile. This immediately frees up memory for other applications to use.

In contrast, paging files function by moving "pages" of a program from system memory into the paging file. These pages are 4KB in size. The entire program does not get swapped wholesale into the paging file.

While swapping occurs when there is heavy demand on the system memory, paging can occur preemptively. This means that the operating system can page out parts of a program when it is minimized or left idle for some time. The memory used by the paged-out portions are not immediately released for use by other applications. Instead, they are kept on standby.

If the paged-out application is reactivated, it can instantly access the paged-out parts (which are still stored in system memory). But if another application requests for the memory space, then the system memory held by the paged-out data is released for its use. As you can see, this is really quite different from the way a swapfile works.

Swapfiles were used in old iterations of Microsoft Windows, prior to Windows 95. From Windows 95 onwards, all Windows versions use only paging files. Therefore, the correct term for the file used to create virtual memory in current operating systems is paging file, not swapfile.

Because both swapfiles and paging files do the same thing - create virtual memory, people will always refer to swapfiles and paging files interchangeably. Let's just keep in mind their innate differences.

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