Adding integers from 2 arrays using Vector takes longer time than traditional for loop

Question

I am trying to use Vector to add integer values from 2 arrays faster than a traditional for loop.

My Vector count is: 4 which should mean that the addArrays_Vector function should run about 4 times faster than: addArrays_Normally

var vectSize = Vector<int>.Count;

This is true on my computer:

Vector.IsHardwareAccelerated

However strangely enough those are the benchmarks:
addArrays_Normally takes 475 milliseconds addArrays_Vectortakes 627 milliseconds

How is this possible? Shouldn't addArrays_Vector take only approx 120 milliseconds? I wonder if I do this wrong?

        void runVectorBenchmark()
        {
            var v1 = new int[92564080];
            var v2 = new int[92564080];
            for (int i = 0; i < v1.Length; i++)
            {
                v1[i] = 2;
                v2[i] = 2;
            }
            
            //new Thread(() => addArrays_Normally(v1, v2)).Start();
            new Thread(() => addArrays_Vector(v1, v2, Vector<int>.Count)).Start();
        }
        void addArrays_Normally(int[] v1, int[] v2)
        {
            Stopwatch stopWatch = new Stopwatch();
            stopWatch.Start();
            int sum = 0;
            int i = 0;
            for (i = 0; i < v1.Length; i++)
            {
                sum = v1[i] + v2[i];
            }
            stopWatch.Stop();
            MessageBox.Show("stopWatch: " + stopWatch.ElapsedMilliseconds.ToString() + " milliseconds\n\n" );
        }
        void addArrays_Vector(int[] v1, int[] v2, int vectSize)
        {
            Stopwatch stopWatch = new Stopwatch();
            stopWatch.Start();
            int[] retVal = new int[v1.Length];
            int i = 0;
            for (i = 0; i < v1.Length - vectSize; i += vectSize)
            {
                var va = new Vector<int>(v1, i);
                var vb = new Vector<int>(v2, i);
                var vc = va + vb;
                vc.CopyTo(retVal, i);
            }
            stopWatch.Stop();
            MessageBox.Show("stopWatch: " + stopWatch.ElapsedMilliseconds.ToString() + " milliseconds\n\n" );
        }

Answer 1

Two functions are different. And looks like RAM memory is a bottleneck here:

• in the first example

      var v1 = new int[92564080];
      var v2 = new int[92564080];
  
      ...
  
      int sum = 0;
      int i = 0;
      for (i = 0; i < v1.Length; i++)
      {
          sum = v1[i] + v2[i];
      }
  

Code is reading both array once. So memory consumption is: sizeof(int) * 92564080 * 2 == 4 * 92564080 * 2 == 706 MB .

• in the second example

      var v1 = new int[92564080];
      var v2 = new int[92564080];
  
      ...            
  
      int[] retVal = new int[v1.Length];
      int i = 0;
      for (i = 0; i < v1.Length - vectSize; i += vectSize)
      {
          var va = new Vector<int>(v1, i);
          var vb = new Vector<int>(v2, i);
          var vc = va + vb;
          vc.CopyTo(retVal, i);
      }
  

Code is reading 2 input arrays and writing into an output array. Memory consumption is at least sizeof(int) * 92564080 * 3 == 1 059 MB

Update:

RAM is much slower than CPU / CPU cache. From this great article about Memory Bandwidth Napkin Math roughly:

L1 Bandwidth: 210 GB/s

...

RAM Bandwidth: 45 GB/s

So extra memory consumption would neglect vectorization speed up.

And the Youtube video mentioned is doing comparison on different code, non-vectorized code from the video is as follows, which consumes the same amount of memory as the vectorized code:

    int[] AddArrays_Simple(int[] v1, int[] v2)
    {
        int[] retVal = new int[v1.Length];
        for (int i = 0; i < v1.Length; i++)
        {
            retVal[i] = v1[i] + v2[i];
        }
        return retVal;
    }