I was writing a few benchmarking tests to figure out why a similar pure algorithm (no C++ lib / .net built in classes) ran much faster in C++ than in C#, even when accounting for the expected feature différences. And while doing so i stumbled on these 2 tests that baffled me, does anyone have an idea about why one is substantially slower thant he other? The only difference in the 2nd one (that takes 51ms vs 88 on my machine) is that the 2 arrays are declared locally in the method instead of outside. In both cases the arrays are created before we start timing.
const int Runs = 100;
const int Width = 5000;
const int Height = 5000;
const int Size = Width * Height;
static int[] Input = Enumerable.Range(0, Size).ToArray();
static int[] Output = new int[Size * 2];
static int SimpleTest()
{
// Removing those 2 lines and using the static arrays instead give substantially slower performance, nearly half the speed!
int[] Input = Enumerable.Range(0, Size).ToArray();
int[] Output = new int[Size * 2];
Stopwatch sw = new Stopwatch();
sw.Start();
for (int run = 0; run < Runs; run++)
{
int InputIndex = 0;
for (int x = 0; x < Width; x++)
{
for (int y = 0; y < Height; y++)
{
int pixel = Input[InputIndex];
var OutputIndex = InputIndex * 2;
Output[OutputIndex] = pixel;
Output[OutputIndex + 1] = pixel;
InputIndex++;
}
}
}
sw.Stop();
return (int)(sw.ElapsedMilliseconds / Runs);
}
