How to Get the Execution Time of a Script
Kernel space vs. User space
A few weeks ago I worked on a fun project for a door monitor with Raspberry Pi 3 model B and shell scripting. I recently connected a camera to the Pi and updated the script so that a picture will be taken when the door is opened and sent as an attached file on the email notification.
One of the issues I ran into was that the execution time of the script increased significantly form few second initially to 17.58 sec. I was able to identify and isolate the execution time of each line on the script. I came to realize that the raspistill command took 5. 87 sec to execute. Which result in missing completely to capture a photo of the person opening the door.
how to reduce the execution time of raspistill
A simple raspistill follow by “ — help” tag on the terminal provided more information on image parameter commands:
pi@raspberrypi:~ $ raspistill --help Runs camera for specific time, and take JPG capture at end if requested usage: raspistill [options]
Image parameter commands
... -t, --timeout : Time (in ms) before takes picture and shuts down (if not specified, set to 5s)
I was able to reduce the execution time of raspistill to few milliseconds by specifying the time before the picture is taken on the script as follow:
How to get the execution time of a script effectively
During that process I learn about the time command, which is used to get the time it takes for another command or script to execute:
time yourscript.sh // get execution for a scripttime ls // get execution time of ls command
which gave the following result:
Now, you can notice that the time command provide a real, user, and system time. Understanding Users space vs Kernel spaces matters. 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, User and Sys process time statistics
- 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 the 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.
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 (and this process is running on a computer with more than one processor) it could potentially exceed the wall clock time reported by Real (which usually occurs). Note that in the output these figures include the User and Sys time of all child processes (and their descendants) as well when they could have been collected, e.g. by wait(2) or waitpid(2), 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 from wait (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 a privileged 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 manipulation of 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 mechanism 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 run 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 library (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.
More 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 it alone can do them. Some operations that you do (like malloc orfread/fwrite) will invoke these Kernel functions and that will count as 'sys' time. Unfortunately, it's not as simple as "every call to malloc will be counted in 'sys' time".
In C, the library function malloc is used to allocate a block of memory on the heap. The program accesses this block of memory via a pointer that malloc returns. When the memory is no longer needed, the pointer is passed to free which deallocates the memory so that it can be used for other purposes.
The call to malloc will do some processing of its own (still counted in 'user' time) and then somewhere along the way it may 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. As for 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.
Some contents of this article are a combination of answers initially posted on Stack Overflow
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