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In computing, wall clock time[1] can also mean 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.

For programs executed in parallel, the CPU time will be the sum of CPU times devoted to the task by each CPU running it. In this case the wall time will be substantially reduced (it takes less perceived time to finish), whereas the total CPU time will remain equal to the one for serial execution (plus some overhead for parallelization).

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. The term provides a way to refer to a time without specifying standard time or daylight saving time.

Wall Clocks Picture
Wall Clocks
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