Why Does My CPU Usage Spike Up and Down?
By Richard Gaughan
In the dark ages of personal computing -- back in the 1980s -- you turned on your computer, typed in some lines, and the end result you were trying to achieve either worked or didn't. Then came the graphical user interface, followed by multitasking, and computer "freezes" became frustrating facts of life because you had no clue as to the source of the problem. Now most computers come with tools for you to monitor performance, including CPU usage, so you can identify problems and properly direct your troubleshooting efforts. But these tools also offer one more layer of frustration when your CPU monitor shows wild variations and you have no idea what's going on. There are a few potential sources for the phenomenon.
The central processing unit of your computer handles the heavy lifting of not only the explicit calculations for spreadsheets and similar applications but also the implicit calculations necessary to perform word processing tasks, video displays, and Web browsing. If you're running multiple applications, each with a fair chunk of CPU usage, your computer will run slowly. Almost all operating systems will allow you to look at your CPU usage to determine how much of the possible CPU capacity is being used by each application. You can also see the overall CPU usage.
Your Program Usage
Obviously, when you initiate an action -- start a calculation, run a video, send an email or start a new program -- you're asking more of your CPU, so the usage will increase. Sometimes, though, you don't ask your computer to do anything new and the CPU usage still jumps up and down. Be aware that even if you don't ask your computer to do anything new the programs you're running can demand more attention. For example, your email program can check the server, your word processor can auto-save your documents and your browser can load a new frame. Those types of requests all ask more of the CPU and will change the CPU usage percentages.
When Nothing Is Running
When nothing is running and your CPU usage still varies -- even when you haven't started any applications and you're running nothing other than the operating system and your CPU monitor -- there are a couple innocuous explanations for this behavior. First, your computer runs background processes such as virus scans and system indexing at regular intervals. Second, most modern processors have the ability to change their speed to save energy. If you're operating at a low level, your processor slows down and the running tasks use a high percentage of the processor capability. If the task is just enough to kick the processor into high gear, the task itself doesn't change, but the CPU has more capability at the higher speed so the CPU usage percentage goes down. Those reasons can send CPU usage swinging with no apparent cause.
The Bad Stuff
Finally, there are some malicious programs out there that can cause your CPU usage to vary while they're up to no good. Viruses and worms of various sorts infiltrate your system specifically so that they can use your CPU. This malware is very good at hiding out in your system, so it appears to be driving your CPU to extremes for no reason. If you suspect this is happening, get yourself a virus-scanning software program as well as a dedicated anti-malware scanner and run them regularly.
First published in 1998, Richard Gaughan has contributed to publications such as "Photonics Spectra," "The Scientist" and other magazines. He is the author of "Accidental Genius: The World's Greatest By-Chance Discoveries." Gaughan holds a Bachelor of Science in physics from the University of Chicago.