I want to measure the running time of a specific system call, for example, I want to know a pread need how many time on both CPU and I/O.
Which function should I use?
Now I usetimes, and it works.
gettimeofday is get the current time, and that may not just calculate the running time of a specific process, right?
clock is return the CPU time this program used so far, does this include the I/O time? If there are other programs running, will this influence the time of this function? I mean something like switching running process.
getrusage seems like a ideal one, but it also returns the CPU time of a specific process.
Does anyone know how benchmark tools like iozone calculate system calls time? I've read its code, and still have no idea.
You're looking for times(2).
Related
Imagine process/thread is running from point A to point B.
I can get how much time the code execution took by taking two gettimeofday() and calculating the difference (wall clock time). However, it may happen, that during the 'route' from A to B CPU was switching to another processes, to drivers, kernel, and other stuff it must perform to keep system running.
Is it possible somehow identify how much time A to B took in terms of actual process/thread execution, and kernel time related to their execution?
The goal for this exercise is to actually identify how much time CPU was NOT executing process/thread or its system calls by executing something else that them.
I am using C.
Searching the man-pages from time (1) backwards I found this:
You can use getrusage:
getrusage() returns resource usage measures
can be used for the own process (self), child processes or the calling thread.
Amongst other values it will give you the user CPU time used and the system CPU time used.
Please see man 2 getrusage for the details. (Or use an online replacement like https://linux.die.net/man/2/getrusage)
I want to measure the total time spent in a C function within Linux. The function may be called at the same time from different threads, and the times spent should be summed together. How can I do this measurement from Linux? I have looked at the clock() function and to calculate the difference between the start and end of the function.
I found one solution using clock() in this thread within Stackoverflow:
How to measure total time spent in a function?
But from what I understand this will also include the CPU processing from threads executes some other function during the time of measurement. Is that a correct assumption?
Is there some other way to do this measurement within Linux?
Your question states that you are using Linux.
You can use the getrusage(2) system call with the RUSAGE_THREAD parameter, which will give you the accumulated statistics for the currently running thread.
By comparing what's in ru_utime, and perhaps ru_stime also, before and after your function runs, you should be able to determine how much time the function has accumulated in CPU time, for the currently running thread.
Lather, rinse, repeat, for all threads, and add them up.
A very good tool for performance analysis is perf (available with recent linux kernels):
Record performance data with
perf record <command>
and then analyze it with
perf report
Compile your program with debug symbols for useful results.
getting time from from clock() and gettimeofday() family functions are good for obtaining precise time difference between two consequent calls, but not good for obtaining time spent in functions, because of thread and process rescheduling of operating system and IO blocking, there isn't any guarantee which your thread/process could obtain CPU until finishes its operations, so you can't relay on time difference.
You have two choice for this
Using profiling softwares such as Intel V-Tune and Intel Inspector which will utilize the hardware performance counters
Using Realtime linux kernel, scheduling your process with FIFO scheduler and use time difference, in FIFO scheduler no one interrupt your program so you can safely use the time difference as time spent in functions, using clock(), gettimeofday() or even more precise rdtsc
I have an assignment where I am analyzing the runtime of various sorting algorithms. I have written the code but I think it's an unfair comparison.
My code basically grabs the the clock time before and after the sorting is finished to compute the elapsed time. However, what if the OS decides to interrupt more frequently during the runtime of a specific sorting algorithm, or if it rather decides that some other background application should be given more of the time domain when it's thread comes back up?
I am not a CS major so I may not be entirely correct here, but from what I've read previously I was concerned this might have an impact on the results.
I also realize that if OS scheduling is suspended and the program hangs then there might be a serious problem; I am just wondering if it possible.
Normally, there's no real reason for it. The scheduler will slightly increase the execution time, but if the code runs for a few seconds, the change will be tiny.
So unless you're running heavy applications on the same computer, the amount of noise this will add to your tests is negligible.
In Linux, you can use isolcpus parameter to mark CPUs that won't be used by the scheduler. You can find information here. I'm not sure what's the minimal kernel version.
If you use it, you'll need to use sched_setaffinity, to put your theread on an isolated CPU, because the scheduler won't put it there.
It is not possible, not in user space code. Otherwise, any malicious process could steal the CPU from others.
If you want precise time counting for your process only, I suggest using time command. You can read about it here: What do 'real', 'user' and 'sys' mean in the output of time(1)?
Quick answer: you are most likely interested in user time, assuming your code doesn't make a heavy use of syscalls (which would be rather strange for a sorting algorithm)
On an up-to-date POSIX system (basically Linux) you can use clock_gettime with CLOCK_PROCESS_CPUTIME_ID or CLOCK_THREAD_CPUTIME_ID if you make sure the process doesn't migrate between CPUs (you can set its affinity for example).
The difference in times returned by clock_gettime with those arguments results in exact time the process/thread spent executing. Only pitfall as I mentioned is process migration as the man page says:
The CLOCK_PROCESS_CPUTIME_ID and CLOCK_THREAD_CPUTIME_ID clocks are realized on many platforms using timers from the CPUs (TSC on i386, AR.ITC on Itanium). These registers may differ between CPUs and as a consequence these clocks may return bogus results if a process is migrated to another CPU.
This means that you don't really need to suspend all other processes just to measure the execution time of your program.
I want to measure the execution time of a c-code segment using Linux.
I take one timestamps at the beginning of the code segment and one at the end.
But I don't know how to protect the code against IRQs and context switches to high prior tasks. The program runs in user space!
The code segment is short so don't panic hosing the system.
Does anyone know an easy solution for this kind of protection?
You can use getrusage(2) to get the CPU time used, rather than just measuring real time. That should get you the answer you want without having to resort to funny business like blocking other programs from running.
As a part of my academic project I have to execute a C program.
I want to get the execution time of the program. For that I have to sleep all other processes in Linux for some seconds. Is there any method for doing that?
(I have tried using the time command in Linux but it is not working properly: it shows different execution time when I am executing the same program. So I am computing execution time by seeing the difference between start time and end time).
About the best way I can think of is to drop to single-user mode, which you get with
# init 1
on pretty much any distribution. This will also stop X, you'll be on a raw console. Handling interrupts from stray mouse movement is likely to be one of the reasons for whatever variability you're seeing, so that's a good thing.
When you want your full system back, init 3 is probably the one, that or init 5.
The usual way to do this is to try to quiesce the machine as much as possible, then take several measurements and average them. It's advisable to discard the first reading, as that's likely to involve population of caches.
It is impossible to get the exact time of execution of a process into a system in which the scheduler commutes from 1 process to the other.
The Intel processors inserted a register that counts the number of clocks, but even so it is impossible to measure the time.
There is a book that you can find as PDF on google, "Computer Systems: A Programmer's Perspective" -- In this book an whole chapter is dedicated to time measurements.
Use the time command. The sum user + sys will give you the time your programm used the CPU directly plus the time the system used the CPU on behalf of your program. I think it is what you want to know.
There will always be a difference in execution time for things no matter how many processes you shut down, polling, IO, background daemons all affect execution priority.
The academic approach would be to run a sizeable sample and take statistics, you might also want to take a look at sar to log the background. To invalidate any readings you might take
Try executing your application with nice -n 20. It may help to make the other processes quieter.
nice man page