程序执行时间信息

Program Time Information

wall clock time(时钟时间): the whole time of a process from start to end.
user CPU time(用户CPU时间): time of execute user instructions. 在用户空间的执行时间。
system CPU time(系统CPU时间): time of program executing in kernel routinue.在内核空间执行内核例程的时间。

These three kind of time is very high level for a process.
We can identify where our program spent most of time. Use time(1)
shell command to get them.

Note: If we want to get time spent of a function or something in this level,
we should use GNU Tools, like gprof.

Example:
$ cd /usr/include/
$ time grep -nri errno ./

real    0m5.571s
user    0m1.470s
sys    0m0.080s