Definitions

clock time

Wall clock time

Succinctly stated, wall clock time is the human perception of the passage of time from the start to the completion of a task. In the context of a task being performed on a computer, wall clock time or wall time is a measure of how much real time that elapses from start to end, including time that passes due to programmed (artificial) delays or waiting for resources to become available.

In computing, wall clock time is the actual time taken by a computer to complete a task. It is the sum of three terms: CPU time, I/O time, and the communication channel delay (e.g. if data are scattered on multiple machines). In contrast to CPU time, which measures only the time during which the processor is actively working on a certain task, wall time measures the total time for the process to complete. The difference between the two consists of time that passes due to programmed delays or waiting for resources to become available.

Consider the example of a mathematical program that reports that it has used "CPU time 0m0.04s, Wall time 6m6.01s". This means that while the program was active for six minutes and six seconds, during that time the computer's processor spent only 4/100 of a second performing calculations for the program.

In the context of time zone calculations, wall clock time identifies the local time with any applicable adjustments for daylight saving time. It provides a way to refer to a time without specifying standard time or daylight saving time.

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