odd behaviour using chrono::high_resolution_clock::now()

Question

I've been looking at various game timing loop methods e.g. Glenn Fiedler and DeWitter. I found critical areas difficult to understand due to my own C++ knowledge limitations. With this I set about trying to implement my own method....I thought a good way to try to understand something about these methods.

[edit1: I'm using CodeBlocks IDE with minGW-w64 (x64-4.8.1-posix-seh-rev5) as the compiler]

[edit2: code and output windows amended to include a 3rd timer, QueryPerformanceCounter]

In trying to accomplish this I came across the following issue:

Minimal code:

#include <chrono>
#include <iostream>
#include <windows.h>
#include <stdio.h>

using namespace std;
using namespace chrono;

LARGE_INTEGER startqpc, stopqpc, li;
double PCFreq = 0.0;


void print()
{
for (int p=0; p<1000000; ) p += 1;  //adjust till ms (steady) returns 1-2ms
}

int main()
{
    for(int x=0; x<200; x += 1)
    {
    steady_clock::time_point start = steady_clock::now();
    auto timePoint1 = chrono::high_resolution_clock::now();

    if(!QueryPerformanceFrequency(&li))
        cout << "QueryPerformanceFrequency failed!\n";
    PCFreq = double(li.QuadPart)/1000.0;
    QueryPerformanceCounter(&startqpc);

    print();

    steady_clock::time_point finish = steady_clock::now();
    auto timePoint2 = chrono::high_resolution_clock::now();
    QueryPerformanceCounter(&stopqpc);
    auto elapsedTime(timePoint2 - timePoint1);
    double diff = (stopqpc.QuadPart - startqpc.QuadPart)/PCFreq;

    cout << "Elapsed time: "
              << duration_cast<milliseconds>(finish - start).count() << "ms (steady)   "
              << chrono::duration_cast<chrono::duration<float, milli >> (elapsedTime).count() << "ms (hires)   "
              << diff << " (qpc) \n";

    }
}

For comparison purposes, the above code makes use of two timers.

On my PC the output from this is as follows:

Elapsed time: 1ms (steady)   0ms (hires)   1.8429 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.8429 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.85152 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   15.6001ms (hires)   1.85557 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.59283 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.25573 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.85072 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84263 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84263 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.85422 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84263 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84263 (qpc)
Elapsed time: 1ms (steady)   15.6001ms (hires)   1.85099 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.31582 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.84964 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.52466 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 2ms (steady)   15.6ms (hires)   2.25169 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84263 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.26974 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84991 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84667 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.89518 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.87874 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.92213 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.87605 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84263 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.03126 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.87659 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.88467 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.24899 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.30316 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23579 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93263 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23552 (qpc)
Elapsed time: 2ms (steady)   15.6ms (hires)   2.42118 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.24522 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23471 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23848 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93425 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.9329 (qpc)
Elapsed time: 1ms (steady)   15.6001ms (hires)   1.85126 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.35355 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23687 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23633 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.2374 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23498 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.85449 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.0097 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.15441 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93263 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93425 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23579 (qpc)
Elapsed time: 2ms (steady)   15.6ms (hires)   2.04554 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.1121 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.01779 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.07222 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.9329 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93425 (qpc)
Elapsed time: 1ms (steady)   15.6001ms (hires)   1.95015 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.95177 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84587 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84479 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84425 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84533 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.85233 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.88278 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84667 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84479 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84533 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.88629 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84506 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.8456 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.8588 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.06225 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23633 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93317 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.06009 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84533 (qpc)
Elapsed time: 1ms (steady)   15.6001ms (hires)   1.86284 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93425 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.02803 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.26839 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93398 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.93641 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93371 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93344 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.9329 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.02345 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.08731 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.9356 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84506 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93398 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.09 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93371 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.03072 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.19213 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.85287 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.9321 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93344 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93317 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93344 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93263 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.42711 (qpc)
Elapsed time: 1ms (steady)   15.6001ms (hires)   1.9418 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.9356 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.08947 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.9329 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93614 (qpc)
Elapsed time: 2ms (steady)   15.6ms (hires)   2.27702 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93614 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93317 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.08974 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.92698 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93317 (qpc)
Elapsed time: 2ms (steady)   15.6ms (hires)   2.09054 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.96066 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.96093 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.8456 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.8456 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.8456 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84533 (qpc)
Elapsed time: 1ms (steady)   15.6001ms (hires)   1.91835 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.8456 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84587 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.8456 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.85233 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84425 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84479 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93317 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93317 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.23552 (qpc)
Elapsed time: 1ms (steady)   15.6ms (hires)   1.93102 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.07033 (qpc)
Elapsed time: 2ms (steady)   0ms (hires)   2.09405 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.9329 (qpc)
Elapsed time: 1ms (steady)   0ms (hires)   1.93263 (qpc)

Process returned 0 (0x0)   execution time : 0.484 s
Press any key to continue.

Using duration_cast<microseconds>(finish - start).count()/1000.0 for the steady_clock we see the following:-

Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84263 (qpc)
Elapsed time: 1.843ms (steady)   0ms (hires)   1.84317 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.846ms (steady)   0ms (hires)   1.84694 (qpc)
Elapsed time: 1.845ms (steady)   0ms (hires)   1.8456 (qpc)
Elapsed time: 1.877ms (steady)   0ms (hires)   1.87685 (qpc)
Elapsed time: 1.916ms (steady)   15.6ms (hires)   1.9162 (qpc)
Elapsed time: 1.906ms (steady)   0ms (hires)   1.90623 (qpc)
Elapsed time: 1.905ms (steady)   0ms (hires)   1.90515 (qpc)
Elapsed time: 1.909ms (steady)   0ms (hires)   1.90919 (qpc)
Elapsed time: 1.905ms (steady)   0ms (hires)   1.90542 (qpc)
Elapsed time: 1.905ms (steady)   0ms (hires)   1.90488 (qpc)
Elapsed time: 1.844ms (steady)   0ms (hires)   1.84452 (qpc)
Elapsed time: 1.943ms (steady)   15.6ms (hires)   1.94287 (qpc)
Elapsed time: 1.872ms (steady)   0ms (hires)   1.87281 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1.846ms (steady)   15.6001ms (hires)   1.8464 (qpc)
Elapsed time: 1.875ms (steady)   0ms (hires)   1.87497 (qpc)
Elapsed time: 2.033ms (steady)   0ms (hires)   2.03449 (qpc)
Elapsed time: 1.922ms (steady)   0ms (hires)   1.92293 (qpc)
Elapsed time: 1.922ms (steady)   0ms (hires)   1.92293 (qpc)
Elapsed time: 1.926ms (steady)   0ms (hires)   1.92698 (qpc)
Elapsed time: 2.004ms (steady)   15.6ms (hires)   2.00485 (qpc)
Elapsed time: 1.847ms (steady)   0ms (hires)   1.84748 (qpc)
Elapsed time: 1.847ms (steady)   0ms (hires)   1.84748 (qpc)
Elapsed time: 1.848ms (steady)   0ms (hires)   1.84856 (qpc)
Elapsed time: 1.849ms (steady)   0ms (hires)   1.84937 (qpc)
Elapsed time: 2.299ms (steady)   0ms (hires)   2.29938 (qpc)
Elapsed time: 3.497ms (steady)   15.6ms (hires)   3.49664 (qpc)
Elapsed time: 3.528ms (steady)   0ms (hires)   3.52843 (qpc)
Elapsed time: 2.299ms (steady)   0ms (hires)   2.29857 (qpc)
Elapsed time: 1.849ms (steady)   0ms (hires)   1.84937 (qpc)
Elapsed time: 1.848ms (steady)   0ms (hires)   1.84856 (qpc)
Elapsed time: 1.848ms (steady)   0ms (hires)   1.84856 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84263 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84263 (qpc)
Elapsed time: 1.854ms (steady)   0ms (hires)   1.85395 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.843ms (steady)   0ms (hires)   1.84371 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.8429 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.843ms (steady)   0ms (hires)   1.8429 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.861ms (steady)   0ms (hires)   1.86123 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84209 (qpc)
Elapsed time: 1.842ms (steady)   0ms (hires)   1.84236 (qpc)
Elapsed time: 2.058ms (steady)   0ms (hires)   2.05848 (qpc)
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Elapsed time: 1.843ms (steady)   0ms (hires)   1.84398 (qpc)
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Elapsed time: 1.85ms (steady)   15.6ms (hires)   1.85099 (qpc)
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Elapsed time: 1.846ms (steady)   15.6ms (hires)   1.8464 (qpc)
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Elapsed time: 1.845ms (steady)   0ms (hires)   1.84506 (qpc)
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Elapsed time: 1.846ms (steady)   15.6ms (hires)   1.84614 (qpc)
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Elapsed time: 1.845ms (steady)   0ms (hires)   1.8456 (qpc)
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Elapsed time: 1.872ms (steady)   15.6001ms (hires)   1.87281 (qpc)
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Elapsed time: 1.845ms (steady)   0ms (hires)   1.84479 (qpc)

Process returned 0 (0x0)   execution time : 0.499 s
Press any key to continue 

Notice how the high resolution clock fluctuates between 0 and 15.6ms and notice the steady_clock comparison.

The issue I have is that is that if I were to implement a fixed-time game loop where the update() code updates entity positions at exactly 1000/60 (60 update cycles a second) i.e. every 16.666etc ms - then this is very close to the returned hires timer fluctuating value! The real problem is that if I also implement an accumulator in order to accommodate both game loop frame times that are under 1000/60ms and any leftover once update() has completed, the update loop is entered sooner - and at irregular time points - than it should.

Is there an issue with the minimal code above, and if not, should I be concerned with this for any proper fixed-time implementation? Additionally, and if relevant, is there any way to isolate a program/thread so that it is not interrupted by system IRQ or other polling events? I'm guessing the hires timer may be subject to such interruptions.

Answer 1

There's a bug in your standard library's implementation of std::chrono.

I'm aware of one such bug in Visual C++, not sure what compiler you are using but if support for std::chrono is a recent addition they might still have some flaws.

If you are using Visual C++, you should be aware of:

• C++ header's high_resolution_clock does not have high resolution
• steady_clock class and clock() function are non conformant with C++11 and C11 standards