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:
https://stackoverflow.com/questions/21470315
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Russian#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)
Elapsed time: 1.842ms (steady) 0ms (hires) 1.84209 (qpc)
Elapsed time: 1.881ms (steady) 0ms (hires) 1.88144 (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: 1.842ms (steady) 0ms (hires) 1.84209 (qpc)
Elapsed time: 1.845ms (steady) 0ms (hires) 1.84479 (qpc)
Elapsed time: 1.843ms (steady) 0ms (hires) 1.84371 (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) 1.84236 (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: 1.896ms (steady) 0ms (hires) 1.89653 (qpc)
Elapsed time: 1.842ms (steady) 0ms (hires) 1.84236 (qpc)
Elapsed time: 1.843ms (steady) 0ms (hires) 1.84398 (qpc)
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Elapsed time: 1.872ms (steady) 15.6001ms (hires) 1.87281 (qpc)
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Process returned 0 (0x0) execution time : 0.499 s
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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.
Solution
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:
