Overhead Considerations

Profiling (compiling with the -pg option) may greatly increase the running time of a program. This is due to the extra overhead required to clock program performance and subprogram calls. Profiling tools like gprof attempt to subtract an approximate overhead factor when computing relative runtime percentages. All other timings shown may not be accurate due to UNIX and hardware timekeeping inaccuracies.

Programs with short execution times are the most difficult to profile because the overhead may be a significant fraction of the total execution time. The best practice is to choose input data for the profiling run that will result in a realistic test of the program's performance. If this is not possible, consider enclosing the main computational part of the program within a loop that effectively runs the program N times. Estimate actual performance by dividing the profile results by N.

The Fortran library includes two routines that return the total time used by the calling process. See the man pages for dtime(3F) and etime(3F).