Parallel.For and For yield different results

Question

If I run this test:

 var r = new Random();
 var ints = new int[13];
 Parallel.For(0, 2000000, i => {            
     var result = r.Next(1, 7) + r.Next(1, 7)         


        

Answer 1

In addition to @spencerruport's assertion that the Random class is not thread safe, your parallel code is also not threadsafe:

 Parallel.For(0, 2000000, i => {            
     //say two threads produce same total at same time
     var result = r.Next(1, 7) + r.Next(1, 7); 
     //what happens on the next line when a context-switch
     //occurs during this non-atomic operation?
     ints[result] += 1;
 });

It might be better to leverage PLINQ to do the collecting of results on your behalf:

Enumerable.Range(0, 2000000)
    .AsParallel()
    .Select(_ => SafeRandom(1, 7) + SafeRandom(1, 7))
    .GroupBy(x => x)
    .Select(g => new {value = g.Key, frequency = g.Count()})

instead of managing access to shared memory (your ints array above) yourself.

A reasonable implementation of SafeRandom might look something like this:

private static int seedUnique=0;
private static ThreadLocal<Random> tlRand=new ThreadLocal<Random>(() => {
    var x=Interlocked.Add(ref seedUnique, 93459872);
    var r=new Random((int)(DateTime.UtcNow.Ticks + x));
    return r;
});
public static int SafeRandom(int min, int max)
{
    return tlRand.Value.Next(min,max);
}

Answer 2

The Random class methods are not thread safe.

http://msdn.microsoft.com/en-us/library/system.random.next(v=vs.90).aspx#2

So the first piece of code is just demonstrating some undefined behavior.

EDIT:

As for a little speculation, from what little I know about operating systems I believe random number generation is a pretty low level operation and hence might even require a context switch. While this is happening you may end up grabbing the same random number multiple times before it's had a chance to update. This would account for the lopsided distribution.

Answer 3

It is thread safety of Random.

I get the below distribution as expected once I've made the call to Random.Next() thread safe.

2: 2.76665
3: 5.5382
4: 8.30805
5: 11.13095
6: 13.8864
7: 16.6808
8: 13.8722
9: 11.14495
10: 8.3409
11: 5.5631
12: 2.76775

public static class Program
{
    private const int Max = 2000000;
    private static readonly object Lock = new object();

    public static void Main()
    {
        var r = new Random();
        var ints = new int[13];
        Parallel.For(0, Max, i =>
        {
            var result = Rand(r, 1, 7) + Rand(r, 1, 7);
            Interlocked.Increment(ref ints[result]);
        });

        for (int i = 0; i < ints.Length; i++)
        {
            Console.WriteLine("{0}: {1}",
                i, ints[i] / ((double)Max) * 100);
        }
    }

    private static int Rand(Random random, int minValue, int maxValue)
    {
        lock (Lock)
        {
            return random.Next(minValue, maxValue);
        }
    }
}