Question

            Stopwatch sw = new Stopwatch();
            for (int i = 0; i < lines.Length; i++)
            {
                sw.Start();
                fn(); //call function
                sw.Stop();

            }
            Console.WriteLine(sw.ElapsedMilliseconds);


            long total =0;
            for (int i = 0; i < lines.Length; i++)
            {
                Stopwatch sw = Stopwatch.StartNew();
                fn(); //call function
                sw.Stop();
                total += sw.ElapsedMilliseconds;

            }
            Console.WriteLine(total);

The output is not the same, do you have any explanation for that?

Answer 1

Leaving aside things like the fact that you’re creating loads of objects in the second loop, which could easily cause garbage collection within fn() or something else to actually make it take longer while timing, you’re also just taking the elapsed milliseconds each iteration in the second case.

Suppose each iteration takes 0.1 milliseconds. Your total for the second loop would be 0, because on each iteration it would round down the elapsed time to 0 milliseconds. The first loop keeps track of the elapsed ticks.

Leaving all this aside, you shouldn’t be starting and stopping the timer this frequently anyway – it will mess with your results. Instead, start the stopwatch once before the loop, and stop it after the loop.

If you want to take out the overhead of the looping, simply time an empty loop to find the overhead, and subtract that from the time taken with a loop containing actual work. Now it’s not really quite that simple, because of the various complexities of real world CPUs – things like cache misses – but microbenchmarking is frankly never particularly accurate in that respect. It should be used as a guide more than anything else.