The central processing unit (CPU) of a computer is where the majority of the low-level calculations take place. CPU time is the amount of time it takes for the CPU to perform its part of the process. This period is part of elapsed time, a figure that includes CPU usage, input and output time, and other computer interactions. Elapsed time covers the entire execution of a program from beginning to end. CPU time used to be a fairly simple calculation, but with the advent of multi-core processors, it has become more complicated.
CPU time measures the time it takes for an operation to finish from the standpoint of the processor. Regardless of the time it takes for the entire operation to execute, CPU time only measures from the point at which it reaches the processor until it finishes with the processor. It is typically measured as time, a percentage of processor capacity, or in clock ticks.
Generally, real-time CPU time measurement is only used when humans need to read the time. Other computers and programs generally find the other methods of determining time more useful. Since most computers don’t think in conventional timeframes, many programs that output time measurement in a real-time amount use one of the other methods and convert those results.
CPU time is expressed as a percentage of capacity when overall processor usage is the goal figure. This is another method of displaying information for a human audience. This method is mostly used to find the total load on a CPU. These figures are used to find peak and lull times on servers or simply to monitor personal computer performance.
Clock ticks are the most difficult CPU time-measurement method for humans to follow, but they are typically the most useful for other computer programs. Clock ticks are a variable amount of time influenced by processor speed. The faster the processor, the faster the clock ticks. These ticks are monitored by other programs to space out processor use and prevent bottlenecks.
Elapsed time, also known as wall clock time or real time, is the entire time it takes a process to execute from beginning to end. With elapsed time, the process is measured from when it begins, regardless of its origin point, to when it ends. This time can involve anything from mouse clicks to CPU processing or disk-access time. Until the advent of multi-core processors, elapsed time was always equal to or greater than CPU time.
Multi-core processors put a twist on CPU time. When using multiple cores, the CPU time is the total time spent on every processor. Since they can receive a single command and split it into multiple processors, it is possible for the total time used to be greater than the total time for the process to execute.