What does it mean when the system monitor reports a program as using over 100 percent CPU?

I've seen this behavior in MacOS's Activity Monitor as well.

An example: Playing Minecraft with OpenJDK. I frequently see over 100 CPU when the game's abusing the system. My girlfriend's MacOS has reported over 100 percent as well on her own programs (not just minecraft).

Solution 1:

100% CPU usage represents full usage of one CPU core.

Anything above 100% represents a multithreaded process utilizing more than one core.

Solution 2:

There are 2 ways to show the CPU usage. In the preference dialog you can choose to between these modes.

The default is IRIX mode which can show a CPU usage of more than 100% on machines with more than 1 logical CPU[1]. More specifically, the maximum CPU usage can be ((# logical CPUs) * 100)%.

In Solaris mode the maximum CPU usage is 100%. Basically this takes the IRIX mode value and divides it by the number of logical CPUs.

This is all consistent with the the top command line tool so see the top man page for more info. http://linux.die.net/man/1/top

[1] If a processor supports hyper-threading then each core can appear as 2 logical CPUs.