What can make a program run slower when using more threads?

Question

This question is about the same program I previously asked about. To recap, I have a program with a loop structure like this:

for (int i1 = 0; i1 < N; i1+         


        

Answer 1

You are seeing cache line bouncing. I'm really surprised that you don't get wrong results, due to race conditions on the histogram buckets.