Why was the parallel version slower than the sequential version in this example?

Question

I've been learning a little about parallelism in the last few days, and I came across this example.

I put it side to side with a sequential for loop like this:

private static void NoParallelTest()
{
    int[] nums = Enumerable.Range(0, 1000000).ToArray();
    long total = 0;
    var watch = Stopwatch.StartNew();
    for (int i = 0; i < nums.Length; i++)
    {
        total += nums[i];
    }
    Console.WriteLine("NoParallel");
    Console.WriteLine(watch.ElapsedMilliseconds);
    Console.WriteLine("The total is {0}", total);
}

I was surprised to see that the NoParallel method finished way way faster than the parallel example given at the site.

I have an i5 PC.

I really thought that the Parallel method would finish faster.

Is there a reasonable explanation for this? Maybe I misunderstood something?

Answer 1

The sequential version was faster because the time spent doing operations on each iteration in your example is very small and there is a fairly significant overhead involved with creating and managing multiple threads.

Parallel programming only increases efficiency when each iteration is sufficiently expensive in terms of processor time.

Answer 2

I think that's because the loop performs a very simple, very fast operation.

In the case of the non-parallel version that's all it does. But the parallel version has to invoke a delegate. Invoking a delegate is quite fast and usually you don't have to worry how often you do that. But in this extreme case, it's what makes the difference. I can easily imagine that invoking a delegate will be, say, ten times slower (or more, I have no idea what the exact ratio is) than adding a number from an array.