Poor Man's Parallel.ForEach Iterator

Introduction

How about a simple parallel ForEach iterator that works with .NET 3.0 until Task Parallel Library is available with .NET 4.0.

Background

Task Parallel Library (TPL) will certainly help developers squeeze every bit of processing power out of their hardware, however Microsoft does not offer TPL for .NET 3.0 or .NET 3.5. TPL and Parallel LINQ will be offered as part of .NET 4.0. Until Microsoft releases .NET 4.0 or developers upgrade their development environments, here is a “Poor Man’s Parallel.ForEach” for the rest of us.

There is vast amount of literature about parallel processing, however in a nutshell I will try to explain the motivation for this code and offer my warnings: Reasonably priced laptops come with two CPU cores and average Windows servers have eight CPU cores. Even if you have ten thousand elements to iterate, a serial iteration such as foreach, while or for can only utilize single core where all the other cores are idling. If a particular procedure doesn't need to access objects that constantly share state, spreading processing over multiple threads provides proven performance gains. Being careful about objects sharing state is utterly important, if a coder blindly starts converting traditional serial iterations to parallel iterations; results will not be any different than giving a chain-saw to an 8 year old.

Using the Code

The usage is of the code is exactly as in TPL. Very simple and concise:

// Sample usage:   
var orders = GetOrders();
   Parallel.ForEach(orders, order => {
                                           if(order.IsValid)
                                             {
                                               order.Process();
                                             }
                                     });

Parallel.ForEach

I used asynchronous delegate invocation. Begin/End pattern is relatively simple to read and maintain. There is possibly an overhead using delegates over using System.Threads. Let me know how you can improve this code. I am looking forward to hear your comments.

public class Parallel
    {
        public static int NumberOfParallelTasks;
        
        static Parallel()
        {
            NumberOfParallelTasks = Environment.ProcessorCount;  
        }

        public static void ForEach<T>(IEnumerable<T> enumerable,Action<T> action)
        {
            var syncRoot = new object();

            if (enumerable == null ) return;

            var enumerator = enumerable.GetEnumerator();
                   
            InvokeAsync<T> del = InvokeAction;
           
            var seedItemArray = new T[NumberOfParallelTasks];
            var resultList = new List<IAsyncResult>(NumberOfParallelTasks);

            for (int i = 0; i < NumberOfParallelTasks; i++)
            {
                bool moveNext;
                
                lock (syncRoot)
                {
                    moveNext = enumerator.MoveNext();
                    seedItemArray[i] = enumerator.Current;
                }
            
                if (moveNext)
                {
                    var iAsyncResult= del.BeginInvoke
		     (enumerator, action, seedItemArray[i], syncRoot,i, null,null);
                    resultList.Add(iAsyncResult);                   
                }               
            } 
            
            foreach(var iAsyncResult in resultList)
            {
                del.EndInvoke(iAsyncResult); 
                iAsyncResult.AsyncWaitHandle.Close();
            }           
        }

        delegate void InvokeAsync<T>(IEnumerator<T> enumerator, 
		Action<T> achtion, T item, object syncRoot,  int i);

        static void InvokeAction<T>(IEnumerator<T> enumerator,Action<T> action,
				T item,object syncRoot,int i )
        {
            if(String.IsNullOrEmpty(Thread.CurrentThread.Name))
                Thread.CurrentThread.Name = 
			String.Format("Parallel.ForEach Worker Thread No:{0}", i);
            
            bool moveNext=true;

            while (moveNext)
            {
                action.Invoke(item);
                
                lock (syncRoot)
                {
                    moveNext = enumerator.MoveNext();
                    item = enumerator.Current;
                }
            }
        } 
    } 

Changes

There is always room for improvement. After publishing this article, I dug up other articles about BeginInvoke. Calling EndInvoke and AsyncWaitHandel.Close() is suggested not to leave garbage collection to chance. I tested it and it works fine and I didn't see performance overhead.

foreach (var iAsyncResult in resultList)iAsyncResult.AsyncWaitHandle.WaitOne();

The above changed to the following code:

foreach(var iAsyncResult in resultList)
            {
                del.EndInvoke(iAsyncResult); 
                iAsyncResult.AsyncWaitHandle.Close();
            }     

History

• 4^th February, 2009: Initial post