What is Virtual Memory?

There's a lot of talk about all things virtual these days: virtual servers, virtual storage, virtual desktops, and so on. Memory can be virtualized as well. So what is virtual memory? When all the programs running on your computer consume the real physical memory (RAM) on your motherboard, your operating system will grab a chunk of unused hard drive space and create virtual memory. Your software can't tell the difference between real memory and virtual memory. The only practical difference is that RAM (real memory) has a much faster access time than a hard drive (virtual memory).

Why is virtual memory useful? Well, unlike physical memory, virtual memory is free and relatively unlimited, as long as you have a few gigabytes of free space on your hard drive. It's a very efficient way to extend the capacity of physical memory. Even though virtual memory is not as fast as real memory, it's better for your computer to manage virtual memory behind the scenes than to say "Sorry, memory is full. Please close one program before opening another."

So how much virtual memory is on your system? The general rule of thumb, at least on Windows-based computers, is for virtual memory to be about 1.5 times the size of the physical memory. But, the more physical memory you have the less virtual memory will need to be used. Here's an example: Let's say you have 2 GB of RAM, and after booting up your computer, and opening your browser and email program, the amount of real memory in use is only 1.8 GB. Your computer will not be using any virtual memory at that point. But suppose you then launch your video editing program and open up a couple of high-res video clips. Now your computer needs 3.4 GB of memory to keep all the balls in the air, so 2 GB of real memory, plus 1.4 GB of virtual memory will be required. If you kept all those programs open and then launched another memory-hungry application, you might approach the combined amount of real plus virtual memory that's available. Only at that point would you see an "Out of Memory" or "Low Virtual Memory" message from your computer.