Why is the CPU usage for a process over 100%?

The CPU percentage (%CPU) displayed by the top command can be misleading on systems with multiple CPU cores due to its different reporting modes.

Irix vs. Solaris Model

The top command has two modes for displaying CPU usage on multi-processor systems, which you can switch between by pressing the I key (Shift+i).

• Irix Mode (On): In this mode, a process's CPU usage is the sum of its usage across all cores. The total can exceed 100%. For example, a process using 100% of one core and 50% of a second core will show a %CPU of 150%. This value represents the total processing power being used in terms of single-core equivalents.
• Solaris Mode (Off): In this mode, a process's total CPU usage is divided by the number of CPU cores. The result is a percentage of the total available CPU power, so the value will not exceed 100%. This is often the default behavior.

Example on a 4-core server:

A process uses 20% on Core 0, 10% on Core 1, 5% on Core 2, and 8% on Core 3.

• Irix Mode On: The reported %CPU is the sum: 20%+10%+5%+8%=43%.
• Solaris Mode On (Irix Off): The reported %CPU is the total usage divided by the number of cores: 43%÷4=10.75%.

For assessing a process's impact on the entire system, Solaris mode (Irix mode off) provides a more intuitive percentage of total capacity.

Threading View

Processes that are multi-threaded are the ones that can utilize multiple cores simultaneously, leading to %CPU values greater than 100% in Irix mode. By default, top shows the combined CPU usage for all threads of a single process.

To see the CPU usage of individual threads, you can press the H key (Shift+h). This toggles "Threads mode," which lists each thread as a separate entry.