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POWhy is array item assignment degrading C# program performance?
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copied!<h3>Summary</h3> <p>While processing a large text file, I came across the following (unexpected) performance degradation that I can't explain. My objectives for this question are:</p> <ul> <li>Understand what causes the slowdown described below</li> <li>Find out how to initialize large non-primitive arrays quickly</li> </ul> <h3>The Problem</h3> <ul> <li>Array contains non-primitive reference items <ul> <li>Not <code>int[]</code> but <code>MyComplexType[]</code></li> <li><code>MyComplexType</code> is a class, not a struct</li> <li><code>MyComplexType</code> contains some <code>string</code> properties</li> </ul></li> <li>Array is pre-allocated</li> <li>Array is large</li> <li>If an item is created and used without being assigned to the array, program is fast</li> <li>If an item is created and then <strong>assigned to an array item</strong>, program <strong>slows down significantly</strong> <ul> <li>I expected array item assignment to be a simple <em>reference assignment</em>, but this does not seem to the the case based on my results for my test program below</li> </ul></li> </ul> <h3>Test program</h3> <p>Consider the following <code>C#</code> program:</p> <pre><code>namespace Test { public static class Program { // Simple data structure private sealed class Item { public Item(int i) { this.Name = "Hello " + i; //this.Name = "Hello"; //this.Name = null; } public readonly string Name; } // Test program public static void Main() { const int length = 1000000; var items = new Item[length]; // Create one million items but don't assign to array var w = System.Diagnostics.Stopwatch.StartNew(); for (var i = 0; i < length; i++) { var item = new Item(i); if (!string.IsNullOrEmpty(item.Name)) // reference the item and its Name property { items[i] = null; // do not remember the item } } System.Console.Error.WriteLine("Without assignment: " + w.Elapsed); // Create one million items and assign to array w.Restart(); for (var i = 0; i < length; i++) { var item = new Item(i); if (!string.IsNullOrEmpty(item.Name)) // reference the item and its Name property { items[i] = item; // remember the item } } System.Console.Error.WriteLine(" With assignment: " + w.Elapsed); } } } </code></pre> <p>It contains two almost-identical loops. Each loop creates one million instances of <code>Item</code> class. First loop uses the created item and then throws away the reference (not persisting it in the <code>items</code> array). Second loop uses the created item and then stores the reference in the <code>items</code> array. Array item assignment is the only difference between the loops.</p> <h3>My Results</h3> <ul> <li><p>When I run <code>Release</code> build (optimizations turned on) on my machine, I get the following results:</p> <pre><code>Without assignment: 00:00:00.2193348 With assignment: 00:00:00.8819170 </code></pre> <p>Loop with array assignment is significantly slower than the one without assignment (~4x slower). </p></li> <li><p>If I change <code>Item</code> constructor to assign a constant string to <code>Name</code> property:</p> <pre><code>public Item(int i) { //this.Name = "Hello " + i; this.Name = "Hello"; //this.Name = null; } </code></pre> <p>I get the following results:</p> <pre><code>Without assignment: 00:00:00.0228067 With assignment: 00:00:00.0718317 </code></pre> <p>Loop with assignment is still ~3x slower than the one without</p></li> <li><p>Finally if I assign <code>null</code> to the <code>Name</code> property:</p> <pre><code>public Item(int i) { //this.Name = "Hello " + i; //this.Name = "Hello"; this.Name = null; } </code></pre> <p>I get the following result:</p> <pre><code>Without assignment: 00:00:00.0146696 With assignment: 00:00:00.0105369 </code></pre> <p>Once no string is allocated, the version without assignment is finally slightly slower (I assume because all those instances are released for garbage collection)</p></li> </ul> <h3>Questions</h3> <ul> <li><p>Why is array item assignment slowing the test program so much?</p></li> <li><p>Is there an attribute/language construct/etc that will speed up the assignment?</p></li> </ul> <p>PS: I tried investigating the slowdown using dotTrace, but it was inconclusive. One thing I saw was a lot more string copying and garbage collection overhead in the loop with assignment than in the loop without assignment (even though I expected the reverse).</p>

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