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POWhy are multi-dimensional arrays in .NET slower than normal arrays?
 

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  1. POWhy are multi-dimensional arrays in .NET slower than normal arrays?
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    <p><strong>Edit:</strong> I apologize everybody. I used the term "jagged array" when I actually meant to say "multi-dimensional array" (as can be seen in my example below). I apologize for using the incorrect name. I actually found jagged arrays to be faster than multi-dimensional ones! I have added my measurements for jagged arrays.</p> <p>I was trying to use a <del>jagged</del> multi-dimensional array today, when I noticed that it's performance is not as I would have expected. Using a single-dimensional array and manually calculating indices was much faster (almost two times) than using a 2D array. I wrote a test using <code>1024*1024</code> arrays (initialized to random values), for 1000 iterations, and I got the following results on my machine:</p> <pre><code>sum(double[], int): 2738 ms (100%) sum(double[,]): 5019 ms (183%) sum(double[][]): 2540 ms ( 93%) </code></pre> <p>This is my test code:</p> <pre><code>public static double sum(double[] d, int l1) { // assuming the array is rectangular double sum = 0; int l2 = d.Length / l1; for (int i = 0; i &lt; l1; ++i) for (int j = 0; j &lt; l2; ++j) sum += d[i * l2 + j]; return sum; } public static double sum(double[,] d) { double sum = 0; int l1 = d.GetLength(0); int l2 = d.GetLength(1); for (int i = 0; i &lt; l1; ++i) for (int j = 0; j &lt; l2; ++j) sum += d[i, j]; return sum; } public static double sum(double[][] d) { double sum = 0; for (int i = 0; i &lt; d.Length; ++i) for (int j = 0; j &lt; d[i].Length; ++j) sum += d[i][j]; return sum; } public static void Main() { Random random = new Random(); const int l1 = 1024, l2 = 1024; double[ ] d1 = new double[l1 * l2]; double[,] d2 = new double[l1 , l2]; double[][] d3 = new double[l1][]; for (int i = 0; i &lt; l1; ++i) { d3[i] = new double[l2]; for (int j = 0; j &lt; l2; ++j) d3[i][j] = d2[i, j] = d1[i * l2 + j] = random.NextDouble(); } // const int iterations = 1000; TestTime(sum, d1, l1, iterations); TestTime(sum, d2, iterations); TestTime(sum, d3, iterations); } </code></pre> <p>Further investigation showed that the IL for the second method is 23% larger than that of the first method. (Code size 68 vs 52.) This is mostly due to calls to <code>System.Array::GetLength(int)</code>. The compiler also emits calls to <code>Array::Get</code> for the <del>jagged</del> multi-dimensional array, whereas it simply calls <code>ldelem</code> for the simple array.</p> <p>So I am wondering, why is access through multi-dimensional arrays slower than normal arrays? I would have assumed the compiler (or JIT) would do something similar to what I did in my first method, but this was not actually the case.</p> <p>Could you plese help me understand why this is happening the way it is?</p> <hr> <p><strong>Update:</strong> Following Henk Holterman's suggestion, here is the implementation of <code>TestTime</code>:</p> <pre><code>public static void TestTime&lt;T, TR&gt;(Func&lt;T, TR&gt; action, T obj, int iterations) { Stopwatch stopwatch = Stopwatch.StartNew(); for (int i = 0; i &lt; iterations; ++i) action(obj); Console.WriteLine(action.Method.Name + " took " + stopwatch.Elapsed); } public static void TestTime&lt;T1, T2, TR&gt;(Func&lt;T1, T2, TR&gt; action, T1 obj1, T2 obj2, int iterations) { Stopwatch stopwatch = Stopwatch.StartNew(); for (int i = 0; i &lt; iterations; ++i) action(obj1, obj2); Console.WriteLine(action.Method.Name + " took " + stopwatch.Elapsed); } </code></pre>
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    1. COA while ago I found a blog post where a matrix inversion (or something like it) was to be optimized. The results proved that jagged arrays where a lot faster than multidimensional arrays. I cannot remember which blog it was though.
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    2. COIt was the B# .NET Blog: http://community.bartdesmet.net/blogs/bart/archive/2007/02/27/c-quiz-need-for-speed.aspx http://community.bartdesmet.net/blogs/bart/archive/2007/03/13/answers-to-c-quiz-need-for-speed.aspx
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    3. COIs this built in release mode with optimizations?
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