Timing algorithm: clock() vs time() in C++

问题:

For timing an algorithm (approximately in ms), which of these two approaches is better:

clock_t start = clock();
algorithm();
clock_t end = clock();
double time = (double) (end-start) / CLOCKS_PER_SEC * 1000.0;

Or,

time_t start = time(0);
algorithm();
time_t end = time(0);
double time = difftime(end, start) * 1000.0;

Also, from some discussion in the C++ channel at Freenode, I know clock has a very bad resolution, so the timing will be zero for a (relatively) fast algorithm. But, which has better resolution time() or clock()? Or is it the same?

回答1:

It depends what you want: time measures the real time while clock measures the processing time taken by the current process. If your process sleeps for any appreciable amount of time, or the system is busy with other processes, the two will be very different.

http://en.cppreference.com/w/cpp/chrono/c/clock

回答2:

<chrono> would be a better library if you're using C++11.

#include <iostream>
#include <chrono>
#include <thread>

void f()
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
}

int main()
{
    auto t1 = std::chrono::high_resolution_clock::now();
    f();
    auto t2 = std::chrono::high_resolution_clock::now();
    std::cout << "f() took "
              << std::chrono::duration_cast<std::chrono::milliseconds>(t2-t1).count()
              << " milliseconds\n";
}

Example taken from here.

回答3:

The time_t structure is probably going to be an integer, which means it will have a resolution of second.

The first piece of code: It will only count the time that the CPU was doing something, so when you do sleep(), it will not count anything. It can be bypassed by counting the time you sleep(), but it will probably start to drift after a while.

The second piece: Only resolution of seconds, not so great if you need sub-second time readings.

For time readings with the best resolution you can get, you should do something like this:

double getUnixTime(void)
{
    struct timespec tv;

    if(clock_gettime(CLOCK_REALTIME, &tv) != 0) return 0;

    return (tv.tv_sec + (tv.tv_nsec / 1000000000.0));
}

double start_time = getUnixTime();
double stop_time, difference;

doYourStuff();

stop_time = getUnixTime();
difference = stop_time - start_time;

On most systems it's resolution will be down to few microseconds, but it can vary with different CPUs, and probably even major kernel versions.

回答4:

<chrono> is the best. Visual Studio 2013 provides this feature. Personally, I have tried all the methods mentioned above. I strongly recommend you use the <chrono> library. It can track the wall time and at the same time have a good resolution (much less than a second).

回答5:

How about gettimeofday? When it is called it updates two structs, with timing information. Usually, the left hand struct is enough, which will carry time since the Epoch, 01-01-1970 00:00:00 (UTC). It can be used as follows:

#include <time.h>

struct timeval start;
double mtime, seconds, useconds;
gettimeofday(&start, NULL); //left hand struct is usually enough
seconds  = start.tv_sec; //time in seconds
useconds = start.tv_usec; //time in microseconds