Limiting fps with std::chrono

Question

std::chrono::system_clock::time_point m_BeginFrame = std::chrono::system_clock::now();
std::chrono::system_clock::time_point m_EndFrame = std::chrono::system_clock::now();
std::chrono::nanoseconds m_WorkTime = std::chrono::nanoseconds::zero();
std::chrono::nanoseconds m_WaitTime = std::chrono::nanoseconds::zero();
auto invFpsLimit = std::chrono::nanoseconds(1e9 / fpsLimit());

// main loop
while (!glfwWindowShouldClose(m_pScreen->glfwWindow()))
{
    m_WaitTime = m_BeginFrame - m_EndFrame;
    m_EndFrame = std::chrono::system_clock::now();
    m_WorkTime = m_EndFrame - m_BeginFrame;

    // if need sleep
    if (m_WorkTime < invFpsLimit)
    {
        std::this_thread::sleep_until(m_BeginFrame + invFpsLimit);
        m_DeltaTime = invFpsLimit;
    }
    else
    {
       m_DeltaTime = m_WorkTime;
    }

    m_BeginFrame = std::chrono::system_clock::now();

    TRACE("   %f   %u   %u   %u",
            ((float)1e9 / (float)(m_WaitTime.count() + m_WorkTime.count())),
            m_WorkTime.count(),
            m_WaitTime.count(),
            invFpsLimit.count());


    // think and opengl stuff...
}

I'm trying to limit fps with this code, but Fraps showes me about 56-57 value

58.848454   3175500   13817300   16666666
55.259304   3314200   14782300   16666666
58.923698   2321700   14649400   16666666
56.200928   2167400   15625900   16666666
52.774071   3188200   15760500   16666666
50.600887   4899700   14862800   16666666
65.011055   2347500   13034500   16666666
54.966499   2611700   15581200   16666666
55.605911   2653400   15330300   16666666
56.476437   2386100   15320400   16666666
55.280689   2581400   15508100   16666666
56.437550   2355400   15363300   16666666
47.170700   5535900   15663700   16666666
64.713608   3039700   12413000   16666666
56.136570   2941100   14872600   16666666
53.444496   3624200   15086800   16666666
60.074493   2397500   14248500   16666666
51.737339   3777100   15551300   16666666
59.369026   3665800   13178000   16666666
59.355282   3543900   13303800   16666666
54.243759   3961000   14474300   16666666
45.764706   7823500   14027400   16666666
61.242237   6239400   10089200   16666666
73.396652   2013800   11610800   16666666
50.531849   3824000   15965500   16666666
62.895454   2796000   13103400   16666666
60.611572   2360500   14138000   16666666
56.074242   2241700   15591800   16666666
55.143208   2454800   15679800   16666666

I have tried to change clock to system_clock, use sleep_for instead of sleep_untill, nothing works
Is there some specific thing I don't khow about std clocks or working code I can't see?
What is stealing my fps?

Answer 1

One can do better by updating m_BeginFrame and m_EndFrame by invFpsLimit instead of by system_clock::now(). This might look something like:

#include <iostream>
#include <thread>

int fpsLimit() {return 60;}

int
main()
{
    using namespace std::chrono;
    using dsec = duration<double>;
    auto invFpsLimit = duration_cast<system_clock::duration>(dsec{1./fpsLimit()});
    auto m_BeginFrame = system_clock::now();
    auto m_EndFrame = m_BeginFrame + invFpsLimit;
    unsigned frame_count_per_second = 0;
    auto prev_time_in_seconds = time_point_cast<seconds>(m_BeginFrame);
    while (true)
    {
        // Do drawing work ...

        // This part is just measuring if we're keeping the frame rate.
        // It is not necessary to keep the frame rate.
        auto time_in_seconds = time_point_cast<seconds>(system_clock::now());
        ++frame_count_per_second;
        if (time_in_seconds > prev_time_in_seconds)
        {
            std::cerr << frame_count_per_second << " frames per second\n";
            frame_count_per_second = 0;
            prev_time_in_seconds = time_in_seconds;
        }

        // This part keeps the frame rate.
        std::this_thread::sleep_until(m_EndFrame);
        m_BeginFrame = m_EndFrame;
        m_EndFrame = m_BeginFrame + invFpsLimit;
    }
}

In C++17 and forward, I recommend constructing invFpsLimit with round instead of duration_cast for slightly better accuracy:

auto invFpsLimit = round<system_clock::duration>(dsec{1./fpsLimit()});

Answer 2

The documentation of sleep_until says:

The function also may block for longer than until after sleep_time has been reached due to scheduling or resource contention delays.

If you really need exact fps use either vSync, or replace the if block in your code with the following loop which continually does nothing until the frame time is used up:

while (m_WorkTime < invFpsLimit)
{
    m_EndFrame = std::chrono::system_clock::now();
    m_WorkTime = m_EndFrame - m_BeginFrame;
}