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POHow to invalidate cache when benchmarking?
 

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  1. POHow to invalidate cache when benchmarking?
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    <p>I have this code, that when swapping the order of UsingAs and UsingCast, their performance also swaps.</p> <pre><code>using System; using System.Diagnostics; using System.Linq; using System.IO; class Test { const int Size = 30000000; static void Main() { object[] values = new MemoryStream[Size]; UsingAs(values); UsingCast(values); Console.ReadLine(); } static void UsingCast(object[] values) { Stopwatch sw = Stopwatch.StartNew(); int sum = 0; foreach (object o in values) { if (o is MemoryStream) { var m = (MemoryStream)o; sum += (int)m.Length; } } sw.Stop(); Console.WriteLine("Cast: {0} : {1}", sum, (long)sw.ElapsedMilliseconds); } static void UsingAs(object[] values) { Stopwatch sw = Stopwatch.StartNew(); int sum = 0; foreach (object o in values) { if (o is MemoryStream) { var m = o as MemoryStream; sum += (int)m.Length; } } sw.Stop(); Console.WriteLine("As: {0} : {1}", sum, (long)sw.ElapsedMilliseconds); } } </code></pre> <p>Outputs:</p> <pre><code>As: 0 : 322 Cast: 0 : 281 </code></pre> <p>When doing this...</p> <pre><code>UsingCast(values); UsingAs(values); </code></pre> <p>...Results to this:</p> <pre><code>Cast: 0 : 322 As: 0 : 281 </code></pre> <p>When doing just this...</p> <pre><code>UsingAs(values); </code></pre> <p>...Results to this:</p> <pre><code>As: 0 : 322 </code></pre> <p>When doing just this:</p> <pre><code>UsingCast(values); </code></pre> <p>...Results to this:</p> <pre><code>Cast: 0 : 322 </code></pre> <p>Aside from running them independently, how to <strong>invalidate</strong> the cache so the second code being benchmarked won't receive the cached memory of first code?</p> <p>Benchmarking aside, just loved the fact that modern processors do this caching magic :-)</p> <p><strong>[EDIT]</strong></p> <p>As advised to try this faster code(supposedly)...</p> <pre><code>static void UsingAsAndNullTest(object[] values) { Stopwatch sw = Stopwatch.StartNew(); int sum = 0; foreach (object o in values) { var m = o as MemoryStream; if (m != null) { sum += (int)m.Length; } } sw.Stop(); Console.WriteLine("As and null test: {0} : {1}", sum, (long)sw.ElapsedMilliseconds); } </code></pre> <p>...The result is this:</p> <pre><code>As and null test: 0 : 342 </code></pre> <p>Slower than the two codes above</p> <p><strong>[EDIT]:</strong></p> <p>As advised to hand each routine their own copy...</p> <pre><code>static void UsingAs(object[] values) { object[] a = values.ToArray(); Stopwatch sw = Stopwatch.StartNew(); int sum = 0; foreach (object o in a) { if (o is MemoryStream) { var m = o as MemoryStream; sum += (int)m.Length; } } sw.Stop(); Console.WriteLine("As: {0} : {1}", sum, (long)sw.ElapsedMilliseconds); } static void UsingCast(object[] values) { object[] a = values.ToArray(); Stopwatch sw = Stopwatch.StartNew(); int sum = 0; foreach (object o in a) { if (o is MemoryStream) { var m = (MemoryStream)o; sum += (int)m.Length; } } sw.Stop(); Console.WriteLine("Cast: {0} : {1}", sum, (long)sw.ElapsedMilliseconds); } </code></pre> <p>...Outputs:</p> <pre><code>Cast: 0 : 282 As: 0 : 282 </code></pre> <p>Now they have the same results, thanks Remus!</p> <p>Running Cast and As independently, they also yield the same result(i.e. 282). Now, as for why they become <strong>faster</strong> (from 322 down to 282 milliseconds) when they are handed their own copy of array, I can't make anything out of it :-) That's entirely another story</p>
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    1. US0x90
    1. USMichael Buen
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    1. COYou know that the fastest way is to not check for `is`, use the `as` operator and test the result for `==null`.
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      1. POHow to invalidate cache when benchmarking?
      1. USRemus Rusanu
    2. COI'm not asking about how to make the code faster. I'm asking how to make proper benchmarking. And I have an inkling that somebody will suggest using `as` and test for `==null` (well somebody have), sad to say, that approach is slower than both of the two codes above
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      1. USMichael Buen
 

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