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POWhy is this faster on 64 bit than 32 bit?
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  1. POWhy is this faster on 64 bit than 32 bit?
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    copied!<p>I've been doing some performance testing, mainly so I can understand the difference between iterators and simple for loops. As part of this I created a simple set of tests and was then totally surprised by the results. For some methods, 64 bit was nearly 10 times faster than 32 bit.</p> <p>What I'm looking for is some explanation for why this is happening. </p> <p>[The answer below states this is due to 64 bit arithmetic in a 32 bit app. Changing the longs to ints results in good performance on 32 and 64 bit systems.]</p> <p>Here are the 3 methods in question.</p> <pre><code>private static long ForSumArray(long[] array) { var result = 0L; for (var i = 0L; i &lt; array.LongLength; i++) { result += array[i]; } return result; } private static long ForSumArray2(long[] array) { var length = array.LongLength; var result = 0L; for (var i = 0L; i &lt; length; i++) { result += array[i]; } return result; } private static long IterSumArray(long[] array) { var result = 0L; foreach (var entry in array) { result += entry; } return result; } </code></pre> <p>I have a simple test harness that tests this</p> <pre><code>var repeat = 10000; var arrayLength = 100000; var array = new long[arrayLength]; for (var i = 0; i &lt; arrayLength; i++) { array[i] = i; } Console.WriteLine("For: {0}", AverageRunTime(repeat, () =&gt; ForSumArray(array))); repeat = 100000; Console.WriteLine("For2: {0}", AverageRunTime(repeat, () =&gt; ForSumArray2(array))); Console.WriteLine("Iter: {0}", AverageRunTime(repeat, () =&gt; IterSumArray(array))); private static TimeSpan AverageRunTime(int count, Action method) { var stopwatch = new Stopwatch(); stopwatch.Start(); for (var i = 0; i &lt; count; i++) { method(); } stopwatch.Stop(); var average = stopwatch.Elapsed.Ticks / count; return new TimeSpan(average); } </code></pre> <p>When I run these, I get the following results:<br> 32 bit:</p> <pre>For: 00:00:00.0006080 For2: 00:00:00.0005694 Iter: 00:00:00.0001717</pre> <p>64 bit</p> <pre>For: 00:00:00.0007421 For2: 00:00:00.0000814 Iter: 00:00:00.0000818</pre> <p>The things I read from this are that using LongLength is slow. If I use array.Length, performance for the first for loop is pretty good in 64 bit, but not 32 bit.</p> <p>The other thing I read from this is that iterating over an array is as efficient as a for loop, and the code is much cleaner and easier to read!</p>
 

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