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When I use the example from MSDN:

var queryA = from num in numberList.AsParallel()
             select ExpensiveFunction(num); //good for PLINQ

var queryB = from num in numberList.AsParallel()
             where num % 2 > 0
             select num; //not as good for PLINQ

My example program:

static void Main(string[] args)
{
   // ThreadPool.SetMinThreads(100, 100);

        var numberList = new List<int>();
        for (int i = 0; i <= 1000; i++)
        {
            numberList.Add(i);
        }
        Stopwatch sw = new Stopwatch();
        sw.Start();
        var queryA = from num in numberList
                     select ExpensiveFunction(num); //good for PLINQ
        var c = queryA.ToList<int>();
        sw.Stop();
        Console.WriteLine(sw.ElapsedMilliseconds);
        sw.Reset();
        sw.Start();
        var queryB = from num in numberList.AsParallel()
                     select ExpensiveFunction(num); //good for PLINQ
        c = queryB.ToList<int>();
        sw.Stop();
        Console.WriteLine(sw.ElapsedMilliseconds);
        Console.ReadKey();
}

static int ExpensiveFunction(int a)
{
    a = a + 100 - 9 + 0 + 98;
    // Console.WriteLine(a);
    return a;
}

The result is: 

7
41

Why is using AsParallel() slower than not using it?

Ry-
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user2532263
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1 Answers1

2

Your ExpensiveFunction really isn't an expensive function for a computer.

Simple maths can be done extremely fast.

Perhaps try Thread.Sleep(500); instead. This will tell the CPU to pause for half a second which will simulate the effect of an actual expensive function.

Edit — I should state, the reason it is slower is because of the overhead parallel processing involves is more work than the actual calculation. See this answer for a better explanation.

Community
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Samuel Parkinson
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