One of the performance indicators is your application timing. Use the time command to provide information about program performance. The following considerations apply to timing your application:
Run program timings when other users are not active. Your timing results can be affected by one or more CPU-intensive processes also running while doing your timings.
Try to run the program under the same conditions each time to provide the most accurate results, especially when comparing execution times of a previous version of the same program. Use the same CPU system (model, amount of memory, version of the operating system, and so on) if possible.
If you do need to change systems, you should measure the time using the same version of the program on both systems, so you know each system's effect on your timings.
For programs that run for less than a few seconds, run several timings to ensure that the results are not misleading. Overhead functions like loading shared libraries might influence short timings considerably.
Using the form of the time command that specifies the name of the executable program provides the following:
The elapsed, real, or "wall clock" time, which will be greater than the total charged actual CPU time.
Charged actual CPU time, shown for both system and user execution. The total actual CPU time is the sum of the actual user CPU time and actual system CPU time.
In the following example timings, the sample program being timed displays the following line:
Average of all the numbers is: 4368488960.000000
Using the Bourne shell, the following program timing reports that the program uses 1.19 seconds of total actual CPU time (0.61 seconds in actual CPU time for user program use and 0.58 seconds of actual CPU time for system use) and 2.46 seconds of elapsed time:
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$ time a.out |
Using the C shell, the following program timing reports 1.19 seconds of total actual CPU time (0.61 seconds in actual CPU time for user program use and 0.58 seconds of actual CPU time for system use), about 4 seconds (0:04) of elapsed time, the use of 28% of available CPU time, and other information:
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% time a.out |
Using the bash shell, the following program timing reports that the program uses 1.19 seconds of total actual CPU time (0.61 seconds in actual CPU time for user program use and 0.58 seconds of actual CPU time for system use) and 2.46 seconds of elapsed time:
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[user@system user]$ time ./a.out |
Timings that show a large amount of system time may indicate a lot of time spent doing I/O, which might be worth investigating.
If your program displays a lot of text, you can redirect the output from the program on the time command line. Redirecting output from the program will change the times reported because of reduced screen I/O.
For more information, see time(1).
In addition to the time command, you might consider modifying the program to call routines within the program to measure execution time. For example, use the Intel Fortran intrinsic procedures, such as SECNDS, DCLOCK, CPU_TIME, SYSTEM_CLOCK, TIME, and DATE_AND_TIME. See "Intrinsic Procedures" in the Intel® Fortran Language Reference.