C++ chrono::duration_cast always outputs “0 seconds”

问题

This is my very first question here and I'm also a complete newbie at C++, but I'll do my best to be as specific as possible. Please tell me if I'm being to vague:

I am trying to measure the time it takes for a sorting method (merge sort) to sort a given array of integers by using chrono and duration_cast. Here is the code snippet in question:

    auto t1 = std::chrono::high_resolution_clock::now();
    mergesort(sortingArray, temp, 0, num - 1);
    auto t2 = std::chrono::high_resolution_clock::now();
    std::chrono::duration<double, std::milli> fp_ms = t2 - t1;
    std::cout << fp_ms.count() << " seconds\n";

And the output I get is always "0 seconds", no matter how big I make the array it needs to sort. Even when it sorts a million integers and there is a noticeable execution time, it still gives me the same output.

I'm basically following the example given here: http://en.cppreference.com/w/cpp/chrono/duration/duration_cast

Only instead of f() I'm using my mergesort function. How can I make it measure my sorting method properly?

EDIT: I'm using minGW to compile via Powershell in Windows 10. The command looks like this:

g++ -std=c++11 .\Merge.cpp

回答1:

TL;DR: It looks like the std::chrono implementation (libstdc++) is quite poor on Windows and you won't get anything better than seconds.

Long version:

libstdc++ typedefs std::chrono::high_resolution_clock to std::chrono::system_clock. According to the implementation a call to std::chrono::system_clock::now() will result in a call to one of the following, depending on the platform:

syscall(SYS_clock_gettime, CLOCK_REALTIME, ...), which is a Linux system call
clock_gettime(CLOCK_REALTIME, ...), which is a POSIX syscall not supported by Windows
gettimeofday(...), which is a POSIX function not supported by Windows
std::time() as a fallback

Thus, std::time() is called internally on Windows. The encoding of std::time() is not specified; however, most systems follow the POSIX specification:

The time() function shall return the value of time in seconds since the Epoch.

Microsoft itself does the same:

Return the time as seconds elapsed since midnight, January 1, 1970, or -1 in the case of an error.

I think it is safe to say that you won't get a higher resolution with MingW's std::chrono.

As for your problem, you have two options:

If your program is running on Windows only you can build your own time measurement using QueryPerformanceCounter
If you want to keep it portable use Boost.Chrono. It uses the native Windows APIs and should offer a better resolution.

来源：https://stackoverflow.com/questions/36913864/c-chronoduration-cast-always-outputs-0-seconds

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