8.2.1 Multiprocessor Interpretation of time Output

Timing results are interpreted in a different way when the program is run in parallel in a multiprocessor environment. Since /bin/time accumulates the user time on different threads, only wall clock time is used.

Since the user time displayed includes the time spent on all the processors, it can be quite large and is not a good measure of performance. A better measure is the real time, which is the wall clock time. This also means that to get an accurate timing of a parallelized program you must run it on a quiet system dedicated to just your program.