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Secrets of Virtual Memory / Swap File

In a multi-tasking Operating System like Windows, one can open as many as applications one needs and work simultaneously on them. For instance, you might be surfing the internet in your web browser and at the same time you can also have your favorite mp3 songs being played in your favorite media player and also in the background have an FTP client uploading your website to he server.

In such situations where you have multiple applications and too much of data occupying the physical memory of the system, quite often the physical memory i.e. RAM in your system becomes insufficient to store all the current data being used. To overcome this physical memory shortage, the operating system makes use of a certain amount of space on the hard drive to substitute for the deficiency of RAM. This part of memory of the hard drive which is used as a substitute for RAM is called the Virtual Memory.

What the operating system does in the virtual memory is, it creates a large file called the swap file which contains all the data which theoretically should have been in the RAM. In Windows, the name of the swap file is Pagefile.sys and can be found as a hidden file in the root of all the drives which have virtual memory allocated in them.

Note here that the difference between swap file and virtual memory is that the virtual memory is the complete block of memory allocated for the purpose of trying to substitute for RAM deficiency and swap file is the amount of the virtual memory currently being used by the operating system in the form of a system file. A swap file may or may not be occupying the entire virtual memory.

Since the swap file is used as a substitute for RAM, the contents of the file are volatile in nature even though the file itself resides in the hard drive. In other words the contents of this file are lost once the system is shutdown, and new contents arrive as soon as your operating system gets loaded. But, finally how the contents of the swap file are handled depends on the architectures of the individual operating systems. By the way, swap file is called so because, the contents of the swap file are always continuously being swapped between the swap file and system RAM. 

It is also to be noted here that even though virtual memory is used as a substitute for system RAM it is in no means as efficient as RAM in terms of speed, as the speed of the virtual memory again depends on that of hard drive as it is located in the hard drive. So, if your system is slower because of inefficient RAM, then to increase its speed increase the physical memory i.e. RAM of the system instead of going for a hard disk with a greater capacity to have more virtual memory.

The performance of your system can come down heavily if the swap file is greatly fragmented in the drives. This usually happens when the operating system itself is managing the virtual memory. A better way out in this case would be to manually allocate a large amount of space on the hard drive for the virtual memory thereby reducing chances of swap file fragmentation. Also, completely disabling the virtual memory in your system may cause your system to slow down heavily and in the worst case might also prevent your system from booting. It is always strongly advised that only technically qualified and knowledgeable persons manage the virtual memory settings. 

-by Gurudev
MADE IN INDIA
gurudevp@vsnl.net
On 08 March 2003

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