26.2. Timing Programs
Two commands, time and /bin/time, provide simple timings. Their information is highly accurate, because no profiling overhead distorts the program's performance. Neither program provides any analysis on the routine or trace level. They report the total execution time, some other global statistics, and nothing more. You can use them on any program.
time and /bin/time differ primarily in that time is built into many shells, including bash. Therefore, it cannot be used in safely portable Bourne shell scripts or in makefiles. It also cannot be used if you prefer the Bourne shell (sh). /bin/time is an independent executable file and therefore can be used in any situation. To get a simple program timing, enter either time or /bin/time, followed by the command you would normally use to execute the program. For example, to time a program named analyze (that takes two command-line arguments, an input file and an output file), enter the following command:
% time analyze inputdata outputfile 9.0u 6.7s 0:30 18% 23+24k 285+148io 625pf+0w
This result (in the default C shell format) indicates that the program spent 9.0 seconds on behalf of the user (user time), 6.7 seconds on behalf of the system (system time, or time spent executing Unix kernel routines on the user's behalf), and a total of 30 seconds elapsed time. Elapsed time is the wall clock time from the moment you enter the command until it terminates, including time spent waiting for other users, I/O time, etc.
By definition, the elapsed time is greater than your total CPU time and can even be several times larger. You can set programs to be timed automatically (without typing time first) or change the output format by setting shell variables.
The example above shows the CPU time as a percentage of the elapsed time (18 percent). The remaining data reports virtual memory management and I/O statistics. The meaning varies, depending on your shell; check your online csh manual page or article.
In this example, under SunOS 4.1.1, the other fields show the amount of shared memory used, the amount of nonshared memory used (k), the number of block input and output operations (io), and the number of page faults plus the number of swaps (pf and w). The memory management figures are unreliable in many implementations, so take them with a grain of salt.
/bin/time reports only the real time (elapsed time), user time, and system time. For example:
% /bin/time analyze inputdata outputfile 60.8 real 11.4 user 4.6 sys
[If you use a shell without a built-in time command, you can just type time. -- JP] This reports that the program ran for 60.8 seconds before terminating, using 11.4 seconds of user time and 4.6 seconds of system time, for a total of 16 seconds of CPU time. On Linux and some other systems, that external time command is in /usr/bin/time and may make a more detailed report.
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