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POJava optimization: speed of inner loops inconsistent?
 

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  1. POJava optimization: speed of inner loops inconsistent?
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    <p>My friend and I are stumped. In these two blocks of code, why is the first inner loop faster than the second inner loop? Is this some sort of JVM optimization?</p> <pre><code>public class Test { public static void main(String[] args) { int[] arr = new int[100000000]; arr[99999999] = 1; long t1, t2, t3; for (int ndx = 0; ndx &lt; 10; ndx++) { t1 = System.currentTimeMillis(); for (int i = 0; i &lt; arr.length; i++) if (0 &lt; arr[i]) System.out.print(""); t2 = System.currentTimeMillis(); for (int i = 0; i &lt; arr.length; i++) if (arr[i] &gt; 0) System.out.print(""); t3 = System.currentTimeMillis(); System.out.println(t2 - t1 +" "+(t3 - t2)); } } } </code></pre> <h3>And the results:</h3> <pre><code>me@myhost ~ $ java Test 57 80 154 211 150 209 149 209 150 209 150 209 151 209 150 210 150 210 149 209 </code></pre> <h2>Swapped the orderings of inequalities:</h2> <pre><code>public class Test { public static void main(String[] args) { int[] arr = new int[100000000]; arr[99999999] = 1; long t1, t2, t3; for (int ndx = 0; ndx &lt; 10; ndx++) { t1 = System.currentTimeMillis(); for (int i = 0; i &lt; arr.length; i++) if (arr[i] &gt; 0) System.out.print(""); t2 = System.currentTimeMillis(); for (int i = 0; i &lt; arr.length; i++) if (0 &lt; arr[i]) System.out.print(""); t3 = System.currentTimeMillis(); System.out.println((t2 - t1) +" "+(t3 - t2)); } } } </code></pre> <h3>And the results:</h3> <pre><code>me@myhost ~ $ java Test 56 80 155 210 150 209 149 209 151 210 149 209 150 209 149 208 149 209 149 208 </code></pre> <p><em>Insanity: doing the same thing over and over again and getting different results.</em> </p>
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    1. USCorey Farwell
    1. USCorey Farwell
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    1. COIt's also interesting that the first pass through the outer loop is _substantially_ faster than subsequent passes (but the ratios between the two are the same). In any case, I'm going to guess that it has to do with what `System.out.print("")` does based on what's already been printed.
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      1. USBrooks Moses
    2. COthe joy of microbenchmarking... you probably want to read this blog - http://www.azulsystems.com/blog/cliff/2011-11-22-what-the-heck-is-osr-and-why-is-it-bad-or-good
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      1. USMatt
    3. COAbout the insanity note: Shouldn't it be "...and _hoping_ to get different results"? Because as it stands, if you really are getting different results each time, it's non-deterministic and it's actually quite reasonable to do the same thing over and over again.
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