In short, the wallclock time elapsed is the real (actual) time that passed during the execution of your program. The CPU time is the time it took for the CPU to execute the program, taking into account the fact that your program may not be the only thing running at the moment, so that more or less this is independent of the load on your system. CPU time is further broken down into time used for system calls and time spent actually in your code.

Using the time Command to Measure CPU Use, despite being a guide for AIX users, has a pretty good explanation as well.

In reply to Re^3: Clocking Portion of Your Perl Code by kaif
in thread Clocking Portion of Your Perl Code by Anonymous Monk

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