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POCompiled C# Lambda Expressions Performance
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  1. POCompiled C# Lambda Expressions Performance
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    copied!<p>Consider the following simple manipulation over a collection:</p> <pre><code>static List&lt;int&gt; x = new List&lt;int&gt;() { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 }; var result = x.Where(i =&gt; i % 2 == 0).Where(i =&gt; i &gt; 5); </code></pre> <p>Now let's use Expressions. The following code is roughly equivalent:</p> <pre><code>static void UsingLambda() { Func&lt;IEnumerable&lt;int&gt;, IEnumerable&lt;int&gt;&gt; lambda = l =&gt; l.Where(i =&gt; i % 2 == 0).Where(i =&gt; i &gt; 5); var t0 = DateTime.Now.Ticks; for (int j = 1; j &lt; MAX; j++) var sss = lambda(x).ToList(); var tn = DateTime.Now.Ticks; Console.WriteLine("Using lambda: {0}", tn - t0); } </code></pre> <p>But I want to build the expression on-the-fly, so here's a new test:</p> <pre><code>static void UsingCompiledExpression() { var f1 = (Expression&lt;Func&lt;IEnumerable&lt;int&gt;, IEnumerable&lt;int&gt;&gt;&gt;)(l =&gt; l.Where(i =&gt; i % 2 == 0)); var f2 = (Expression&lt;Func&lt;IEnumerable&lt;int&gt;, IEnumerable&lt;int&gt;&gt;&gt;)(l =&gt; l.Where(i =&gt; i &gt; 5)); var argX = Expression.Parameter(typeof(IEnumerable&lt;int&gt;), "x"); var f3 = Expression.Invoke(f2, Expression.Invoke(f1, argX)); var f = Expression.Lambda&lt;Func&lt;IEnumerable&lt;int&gt;, IEnumerable&lt;int&gt;&gt;&gt;(f3, argX); var c3 = f.Compile(); var t0 = DateTime.Now.Ticks; for (int j = 1; j &lt; MAX; j++) var sss = c3(x).ToList(); var tn = DateTime.Now.Ticks; Console.WriteLine("Using lambda compiled: {0}", tn - t0); } </code></pre> <p>Of course it isn't exactly like the above, so to be fair, I modify the first one slightly:</p> <pre><code>static void UsingLambdaCombined() { Func&lt;IEnumerable&lt;int&gt;, IEnumerable&lt;int&gt;&gt; f1 = l =&gt; l.Where(i =&gt; i % 2 == 0); Func&lt;IEnumerable&lt;int&gt;, IEnumerable&lt;int&gt;&gt; f2 = l =&gt; l.Where(i =&gt; i &gt; 5); Func&lt;IEnumerable&lt;int&gt;, IEnumerable&lt;int&gt;&gt; lambdaCombined = l =&gt; f2(f1(l)); var t0 = DateTime.Now.Ticks; for (int j = 1; j &lt; MAX; j++) var sss = lambdaCombined(x).ToList(); var tn = DateTime.Now.Ticks; Console.WriteLine("Using lambda combined: {0}", tn - t0); } </code></pre> <p>Now comes the results for MAX = 100000, VS2008, debugging ON:</p> <pre><code>Using lambda compiled: 23437500 Using lambda: 1250000 Using lambda combined: 1406250 </code></pre> <p>And with debugging OFF:</p> <pre><code>Using lambda compiled: 21718750 Using lambda: 937500 Using lambda combined: 1093750 </code></pre> <p><em>Surprise</em>. The compiled expression is roughly 17x slower than the other alternatives. Now here comes the questions:</p> <ol> <li>Am I comparing non-equivalent expressions?</li> <li>Is there a mechanism to make .NET "optimize" the compiled expression?</li> <li>How do I express the same chain call <code>l.Where(i =&gt; i % 2 == 0).Where(i =&gt; i &gt; 5);</code> programatically?</li> </ol> <hr> <p>Some more statistics. Visual Studio 2010, debugging ON, optimizations OFF:</p> <pre><code>Using lambda: 1093974 Using lambda compiled: 15315636 Using lambda combined: 781410 </code></pre> <p>Debugging ON, optimizations ON:</p> <pre><code>Using lambda: 781305 Using lambda compiled: 15469839 Using lambda combined: 468783 </code></pre> <p>Debugging OFF, optimizations ON:</p> <pre><code>Using lambda: 625020 Using lambda compiled: 14687970 Using lambda combined: 468765 </code></pre> <p><em>New Surprise.</em> Switching from VS2008 (C#3) to VS2010 (C#4), makes the <code>UsingLambdaCombined</code> faster than the native lambda.</p> <hr> <p>Ok, I've found a way to improve the lambda compiled performance by more than an order of magnitude. Here's a tip; after running the profiler, 92% of the time is spent on:</p> <pre><code>System.Reflection.Emit.DynamicMethod.CreateDelegate(class System.Type, object) </code></pre> <p>Hmmmm... Why is it creating a new delegate in every iteration? I'm not sure, but the solution follows in a separate post.</p>
 

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