CPU time (or process time) is the amount of time that a central processing unit (CPU) was used for processing a specific set of instructions of a computer program or operating system, as opposed to total time from start to finish, where the CPU might have spent time on other processes. See the figure. CPU time is measured in clock ticks or seconds. Sometimes it is useful to convert CPU time into a percentage of the CPU capacity, giving the CPU usage.
Measuring CPU time for two functionally identical programs that process identical inputs can indicate which program is faster, but it is a common misunderstanding that CPU time can be used to compare algorithms. Comparing programs by their CPU time compares specific implementations of algorithms. (It is possible to have both efficient and inefficient implementations of the same algorithm.) Algorithms are more commonly compared using measures of time complexity and space complexity.