Measuring the runtime of a C++ code?

Question

I want to measure the runtime of my C++ code. Executing my code takes about 12 hours and I want to write this time at the end of execution of my code. How can I do it in my code?

Operating system: Linux

Answer 1

Use std::chrono::steady_clock and not std::chrono::system_clock for measuring run time in C++11. The reason is (quoting system_clock's documentation):

on most systems, the system time can be adjusted at any moment

while steady_clock is monotonic and is better suited for measuring intervals:

Class std::chrono::steady_clock represents a monotonic clock. The time points of this clock cannot decrease as physical time moves forward. This clock is not related to wall clock time, and is best suitable for measuring intervals.

Here's an example:

auto start = std::chrono::steady_clock::now();
// do something
auto finish = std::chrono::steady_clock::now();
double elapsed_seconds = std::chrono::duration_cast<
  std::chrono::duration<double>>(finish - start).count();

A small practical tip: if you are measuring run time and want to report seconds std::chrono::duration_cast<std::chrono::seconds> is rarely what you need because it gives you whole number of seconds. To get the time in seconds as a double use the example above.

Answer 2

If you wish to print the measured time with printf(), you can use this:

auto start = std::chrono::system_clock::now();
/* measured work */
auto end = std::chrono::system_clock::now();
auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(end - start);
printf("Time = %lld ms\n", static_cast<long long int>(elapsed.count()));

Answer 3

This is the code I use:

  const auto start = std::chrono::steady_clock::now();

  // Your code here.

  const auto end = std::chrono::steady_clock::now();
  std::chrono::duration<double> elapsed = end - start;
  std::cout << "Time in seconds: " << elapsed.count() << '\n';

You don't want to use std::chrono::system_clock because it is not monotonic! If the user changes the time in the middle of your code your result will be wrong - it might even be negative. std::chrono::high_resolution_clock might be implemented using std::chrono::system_clock so I wouldn't recommend that either.

This code also avoids ugly casts.

Answer 4

You could also try some timer classes that start and stop automatically, and gather statistics on the average, maximum and minimum time spent in any block of code, as well as the number of calls. These cxx-rtimer classes are available on GitHub, and offer support for using std::chrono, clock_gettime(), or boost::posix_time as a back-end clock source.

With these timers, you can do something like:

void timeCriticalFunction() {
    static rtimers::cxx11::DefaultTimer timer("expensive");
    auto scopedStartStop = timer.scopedStart();
    // Do something costly...
}

with timing stats written to std::cerr on program completion.

Answer 5

I used something like this in one of my projects:

#include <sys/time.h>

struct timeval start, end;
gettimeofday(&start, NULL);
//Compute
gettimeofday(&end, NULL);
double elapsed = ((end.tv_sec - start.tv_sec) * 1000) 
        + (end.tv_usec / 1000 - start.tv_usec / 1000);

This is for milliseconds and it works both for C and C++.

Answer 6

If you are using C++11 you can use system_clock::now():

auto start = std::chrono::system_clock::now();

/* do some work */

auto end = std::chrono::system_clock::now();
auto elapsed = end - start;
std::cout << elapsed.count() << '\n';

You can also specify the granularity to use for representing a duration:

// this constructs a duration object using milliseconds
auto elapsed =
    std::chrono::duration_cast<std::chrono::milliseconds>(end - start);

// this constructs a duration object using seconds
auto elapsed =
    std::chrono::duration_cast<std::chrono::seconds>(end - start);

If you cannot use C++11, then have a look at chrono from Boost.

The best thing about using such a standard libraries is that their portability is really high (e.g., they both work in Linux and Windows). So you do not need to worry too much if you decide to port your application afterwards.

These libraries follow a modern C++ design too, as opposed to C-like approaches.

EDIT: The example above can be used to measure wall-clock time. That is not, however, the only way to measure the execution time of a program. First, we can distinct between user and system time:

• User time: The time spent by the program running in user space.
• System time: The time spent by the program running in system (or kernel) space. A program enters kernel space for instance when executing a system call.

Depending on the objectives it may be necessary or not to consider system time as part of the execution time of a program. For instance, if the aim is to just measure a compiler optimization on the user code then it is probably better to leave out system time. On the other hand, if the user wants to determine whether system calls are a significant overhead, then it is necessary to measure system time as well.

Moreover, since most modern systems are time-shared, different programs may compete for several computing resources (e.g., CPU). In such a case, another distinction can be made:

• Wall-clock time: By using wall-clock time the execution of the program is measured in the same way as if we were using an external (wall) clock. This approach does not consider the interaction between programs.
• CPU time: In this case we only count the time that a program is actually running on the CPU. If a program (P1) is co-scheduled with another one (P2), and we want to get the CPU time for P1, this approach does not include the time while P2 is running and P1 is waiting for the CPU (as opposed to the wall-clock time approach).

For measuring CPU time, Boost includes a set of extra clocks:

• process_real_cpu_clock, captures wall clock CPU time spent by the current process.
• process_user_cpu_clock, captures user-CPU time spent by the current process.
• process_system_cpu_clock, captures system-CPU time spent by the current process. A tuple-like class process_cpu_clock, that captures real, user-CPU, and system-CPU process times together.
• A thread_clock thread steady clock giving the time spent by the current thread (when supported by a platform).

Unfortunately, C++11 does not have such clocks. But Boost is a wide-used library and, probably, these extra clocks will be incorporated into C++1x at some point. So, if you use Boost you will be ready when the new C++ standard adds them.

Finally, if you want to measure the time a program takes to execute from the command line (as opposed to adding some code into your program), you may have a look at the time command, just as @BЈовић suggests. This approach, however, would not let you measure individual parts of your program (e.g., the time it takes to execute a function).

Answer 7

You can use time to start your program. When it ends, it print nice time statistics about program run. It is easy to configure what to print. By default, it print user and CPU times it took to execute the program.

EDIT : Take a note that every measure from the code is not correct, because your application will get blocked by other programs, hence giving you wrong values^*.

^{* By wrong values, I meant it is easy to get the time it took to execute the program, but that time varies depending on the CPUs load during the program execution. To get relatively stable time measurement, that doesn't depend on the CPU load, one can execute the application using time and use the CPU as the measurement result.}