Comparing the Performance of Various Iteration Methods...

When you have to iterate through large amounts of data, you can save significant time by choosing your iteration method carefully.

By: Trevor Misfeldt Date: April 17, 2003 Download the code.

Introduction

I’ve been implementing numerical libraries in .NET and have come to some conclusions about iteration performance. My classes have to hold a large amount of data and be able to iterate through that data as quickly as possible. In order to compare various methods, I created a simple class called Data that encapsulates an array of doubles.

Method #1: Enumeration

Data implements IEnumerable. It contains GetEnumerator which returns its own DataEnumerator, an inner class.

...
   public IEnumerator GetEnumerator()
    {
      return new DataEnumerator( this );
    }

    internal class DataEnumerator : IEnumerator
    {
      private Data internal_ = null;
      private int index = -1;

      public DataEnumerator( Data data )
      {
        internal_ = data;
      }

      public object Current
      {
        get
        {
          return internal_.Array[index];
        }
      }

      public bool MoveNext()
      {
        index++;
        if ( index >= internal_.Array.Length )
        {
          return false;
        }
        return true;
      }

      public void Reset()
      {
        index = -1;
      }
    }
...

Method #2: Indexing

I implemented an index operator on the class which simply calls the index operator on the array.

    public double this[int position]
    {
      get
      {
        return array_[position];
      }
    }

Method #3: Indirect Array

I created a property to access the array.

    public double[] Array
    {
      get
      {
        return array_;
      }
    }

When iterating, I called the Array property and then its index operator.

d = data.Array[j];

Method #3: Direct Array

I created a reference to the array.

double[] array = data.Array;

Then, I iterate through that reference.

d = array[j];

Method #4: Pointer Math

Finally, I tried improving performance by iterating through the array in Managed C++ using pointer manipulation.

    static void iterate( Data& data )
    {
      double d;
      double __pin* ptr = &( data.Array[0] );
      for ( int i = 0; i < data.Array.Length; i++ )
      {
        d = *ptr;
        ++ptr;
      }
    }

I called it this way:

Pointer.iterate( data );

Conclusions

To test the different methods, I allocated 1,000,000 doubles into an array and indexed over all of them. I repeated this 1,000 times to minimize randomness. Here are the results...

Enumeration is always slow. That’s not surprising as I’m using a general data structure to hold the doubles. Each access performs a cast. The three operator/property methods differed very slightly. These are probably all optimized similarly. Using pointer math to traverse over the raw data was significantly faster. This is probably due to the fact that there’s no bounds checking. In summary, if you have large amounts of data and performance is critical, consider using managed C++.

Acknowledgements

Thanks to Mark Vulfson of ProWorks for tips on using Flipper Graph Control. Also, to my colleagues Ken Baldwin and Steve Sneller at CenterSpace Software.

You may download the code here.