《APUE》上提到了三個時間Real time, User time和Sys time。這三者是什麼關係呢？在使用time(1)命令的時候，爲什麼real time < user time + sys time？（由於筆者的虛擬機爲單核，顯示的結果不會出現此種情況。附網上看到的一種結果如下：
- $ time foo
- real 0m0.003s
- user 0m0.000s
- sys 0m0.004s
- $
Real指的是實際經過的時間，User和Sys指的是該進程使用的CPU時間。
1. Real是牆上時間(wall clock time)，也就是進程從開始到結束所用的實際時間。這個時間包括其他進程使用的時間片和進程阻塞的時間（比如等待I/O完成）。
2. User指進程執行用戶態代碼（核心之外）所使用的時間。這是執行此進程所消耗的實際CPU時間，其他進程和此進程阻塞的時間並不包括在內。
3. Sys指進程在內核態消耗的CPU時間，即在內核執行系統調用所使用的CPU時間。
那麼，爲什麼進程開始到結束所經過的時間會比進程所消耗的用戶時間和系統時間(user time + sys time)小呢？
User+Sys爲進程所使用的實際CPU時間。注意，如果有多個線程，User+Sys的時間有可能大於Real時間。同時，User和Sys時間包括子進程所使用的時間。
time命令的輸出數據是由幾個不同的系統調用得來的。User time和Sys time從wait(2)或times(2)系統調用（依賴不同的系統）得來。Real time是由gettimeofday(2)中結束時間和起始時間相減得到。不同的操作系統還可能有其他的信息，比如time可以記錄上下文切換的次數。
在多處理器的系統上，一個進程如果有多個線程或者有多個子進程可能導致Real time比CPU time（User + Sys time）要小，這是因爲不同的線程或進程可以並行執行。
參考：點擊打開鏈接
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.
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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).
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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.
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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.
User+Sys will tell you how much actual CPU time your process used. Note that this is across all CPUs, so if the process has multiple threads it could potentially exceed the wall clock time reported by Real. Note that in the output
these figures include the User and Sys time of all child processes as well, although the underlying system calls return the statistics for the process and its children separately.
Origins of the statistics reported by time (1)
The statistics reported by time are gathered from various system calls. 'User' and 'Sys' come fromwait
(2) or times (2), depending on the particular system. 'Real' is calculated from a start and end time gathered from the gettimeofday
(2) call. Depending on the version of the system, various other statistics such as the number of context switches may also be gathered by time.
On a multi-processor machine a multi-threaded process or a process forking children could have an elapsed time smaller than the total CPU time - as different threads or processes may run in parallel. Also, the time statistics reported come from different origins, so times recorded for very short running tasks may be subject to rounding errors, as the example given by the original poster shows.
A brief primer on Kernel vs. User mode
On unix, or any protected-memory operating system, 'Kernel' or 'Supervisor' mode refers to aprivileged mode that the CPU can operate in. Certain privileged actions that could affect security or stability can only be done when the CPU is operating in this mode; these actions are not available to application code. An example of such an action might be to manipulate the MMU to gain access to the address space of another process. Normally, user-mode code cannot do this (with good reason), although it can request shared memory from the kernel, which could be read or written by more than one process. In this case, the shared memory is explicitly requested from the kernel through a secure mechansm and both processes have to explicitly attach to it in order to use it.
The privileged mode is usually referred to as 'kernel' mode because the kernel is executed by the CPU running in this mode. In order to switch to kernel mode you have to issue a specific instruction (often called a trap) that switches the CPU to running in kernel mode and runs code from a specific location held in a jump table. For security reasons, you cannot switch to kernel mode and execute arbitrary code - the traps are managed through a table of addresses that cannot be written to unless the CPU is running in supervisor mode. You trap with an explicit trap number and the address is looked up in the jump table; the kernel has a finite number of controlled entry points.
The 'system' calls in the C libary (particularly those described in Section 2 of the man pages) have a user-mode component, which is what you actually call from your C program. Behind the scenes they may issue one or more system calls to the kernel to do specific services such as I/O, but they still also have code running in user-mode. It is also quite possible to directly issue a trap to kernel mode from any user space code if desired, although you may need to write a snippet of assembly language to set up the registers correctly for the call. A page describing the system calls provided by the Linux kernel and the conventions for setting up registers can be found here.
Added by Vilx-
To clarify about 'sys': There are things that your code cannot do from user mode - things like allocating memory or accessing hardware (HDD, network, etc.) These are under the supervision of The Kernel, and he alone can do them. Some operations that you do (like malloc or fread/fwrite) will invoke these Kernel functions and that then will count as 'sys' time. Unfortunately it's not as simple as "every call to malloc will be counted in 'sys' time". The call to malloc will do some processing of its own (still counted in 'user' time) and then somewhere along the way call the function in kernel (counted in 'sys' time). After returning from the kernel call there will be some more time in 'user' and then malloc will return to your code. When the switch happens and how much of it is spent in kernel mode - you cannot say. It depends on the implementation of the library. Also, other seemingly innocent functions might also use malloc and the like in the background, which will again have some time in 'sys' then.
