Wallclock time

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The Wallclock time (also sometimes called real time, as for instance by /bin/time) for a program to run is the actual time that elapses (eg, according to the clock on the wall) during the run. This may differ from CPU time for the program even on a single processor system, as during the elapsed wallclock time the CPU may be spending some of its time on other tasks than the program the user is running; thus the wallclock time would be greater than the CPU time spent on the processes. For jobs run in parallel computing environments, the total CPU time for a job may be much greater than the wallclock time, as many CPUs may be spending time on the task at any given moment.