Real, User and Sys process time statistics

One of these things is not like the other. Real refers to actual elapsed time; User and Sys refer to CPU time used only by the process.

*Real is wall clock time - time from start to finish of the call. This is all elapsed time including time slices used by other processes and time the process spends blocked (for example if it is waiting for I/O to complete).

*User is the amount of CPU time spent in user-mode code (outside the kernel) within the process. This is only actual CPU time used in executing the process. Other processes and time the process spends blocked do not count towards this figure.

*Sys is the amount of CPU time spent in the kernel within the process. This means executing CPU time spent in system calls within the kernel, as opposed to library code, which is still running in user-space. Like 'user', this is only CPU time used by the process. See below for a brief description of kernel mode (also known as 'supervisor' mode) and the system call mechanism.

Linux中time命令，我们经常用来计算某个程序的运行耗时，用户态cpu耗时，系统态cpu耗时。

例如：

$ time foo
real        0m0.003s
user        0m0.000s
sys         0m0.004s$

那么这三个时间都具体代表什么意思呢？

[1] real : 表示foo程序整个的运行耗时，可以理解为foo运行开始时刻你看了一下手表，foo运行结束时，你又看了一下手表，两次时间的差值就是本次real 代表的值

举个极端的