How to get execution time of a piece of code in C++?

Question

I'm trying to benchmark a piece of code. It is based on this code here with the interesting part basically looking like this:

 auto t0 = std::chrono::high_resolution_clock::now(); ... auto t1 = std::chrono::high_resolution_clock::now(); std::cout << secs(t1-t0).count() << " s\n";

The problem is that I am on a shared machine, and this is giving me wall time, so my results are not giving me anything consistent. What I really need is some sort of stopwatch tool that will allow me get just the time in which my code is running, rather than the wall time. What options are there? I'm on a Linux machine if it comes to system specific calls, but I'd rather keep this code portable if possible.

I've looked at other questions such as this and this but they all seem to offer wall time solutions.

Have you considered isolating the code in main, compiling, and running it via time? (Not the best for measuring small pieces of code, but since you're using seconds, I assume this should be good enough) — user123
– user123, Commented Jul 3, 2014 at 18:35
I could do that as a last resort, however I'm running a ton of tests with a ton of configuration, and if I have to keep copying into main, this testing will take a lot longer and will be a lot less pleasant — GBleaney
– GBleaney, Commented Jul 3, 2014 at 18:39
When you say "time", do you mean CPU time? If you were to exclude all other process's use of the CPU, would you want the time to include i/o? — wallyk
– wallyk, Commented Jul 3, 2014 at 18:40
There is no I/O time in this particular case. Its just small snippets of code that are testing the behavior of an unordered map with different algorithms and different input. — GBleaney
– GBleaney, Commented Jul 3, 2014 at 19:00
I was expecting to see some mention of gprof and gcc flags to make it work. Why not use a profiler for this? See questions like stackoverflow.com/questions/21072058/…. — wojciii
– wojciii, Commented Jul 3, 2014 at 21:55

KillianDS · Accepted Answer · 2014-07-03 18:48:37Z

4

If available on your OS you can use the times API to get similar results as the built in time command.

#include <sys/times.h> ... struct tms start, end; times(&start); //Do command times(&end); clock_t usr_time = end->tms_utime - start->tms_utime; clock_t sys_time = end->tms_stime - start->tms_stime;

To be 100% complete you should check that the result of times is not equal to -1 and check the errno code otherwise.

answered Jul 3, 2014 at 18:48

KillianDS

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3 Comments

joesdiner Over a year ago

There is a typo in the answer. start and end are not pointers so the calculations should be clock_t usr_time = end.tms_utime - start.tms_utime; and clock_t sys_time = end.tms_stime - start.tms_stime;

joesdiner Over a year ago

Also, the units returned by times() is the number of elapsed ticks as per CLOCKS_PER_SEC. So to get the number of seconds of elapsed time do (double) usr_time / CLOCKS_PER_SEC (I think).

Alex M.M. Over a year ago

Kilian please take the time to correct the above typo.

Roy Longbottom · Accepted Answer · 2014-07-03 20:12:52Z

For Linux

 #include <time.h> #include <stdio.h> double theseSecs = 0.0; double startSecs = 0.0; double secs; double CPUsecs = 0.0; double CPUutilisation = 0.0; double answer = 0; clock_t starts; void start_CPU_time() { starts = clock();; return; } void end_CPU_time() { CPUsecs = (double)(clock() - starts)/(double)CLOCKS_PER_SEC; return; } struct timespec tp1; void getSecs() { clock_gettime(CLOCK_REALTIME, &tp1); theseSecs = tp1.tv_sec + tp1.tv_nsec / 1e9; return; } void start_time() { getSecs(); startSecs = theseSecs; return; } void end_time() { getSecs(); secs = theseSecs - startSecs; return; } void calculate() { int i, j; for (i=1; i<100001; i++) { for (j=1; j<10001; j++) { answer = answer + (float)i / 100000000.0; } } } void main() { start_time(); start_CPU_time(); calculate(); end_time(); end_CPU_time(); CPUutilisation = CPUsecs / secs * 100.0; printf("\n Answer %10.1f, Elapsed Time %7.4f, CPU Time %7.4f, CPU Ut %3.0f%\n", answer, secs, CPUsecs, CPUutilisation); }

merlin2011 · Accepted Answer · 2014-07-03 18:43:36Z

Ideally, as the comment under your question mentioned, you should isolate the piece of code you are trying to measure inside main and just use the unix time command line tool, which reports user and system time.

If you are unable to do that, consider looking at the source code for time and potentially using the same technique to benchmark your code.

Community · Accepted Answer · 2017-05-23 11:59:33Z

I think you need to use clock_gettime() with the following clocks CLOCK_PROCESS_CPUTIME_ID or CLOCK_THREAD_CPUTIME_ID. They give amount of CPU time consumed by a process/thread.

CLOCK_PROCESS_CPUTIME_ID High-resolution per-process timer from the CPU. CLOCK_THREAD_CPUTIME_ID Thread-specific CPU-time clock.

Reference:

I think the advice that gives merlin2011 is infeasible in many cases since isolating might take lots of efforts while adding two lines of code with clock_gettime() is quite practical. You even can wrap calls to clock_gettime() in a class.

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