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PODeclaring multiple arrays with 64 elements 1000 times faster than declaring array of 65 elements
 

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  1. PODeclaring multiple arrays with 64 elements 1000 times faster than declaring array of 65 elements
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    <p>Recently I noticed declaring an array containing 64 elements is a lot faster (>1000 fold) than declaring the same type of array with 65 elements.</p> <p>Here is the code I used to test this:</p> <pre><code>public class Tests{ public static void main(String args[]){ double start = System.nanoTime(); int job = 100000000;//100 million for(int i = 0; i &lt; job; i++){ double[] test = new double[64]; } double end = System.nanoTime(); System.out.println("Total runtime = " + (end-start)/1000000 + " ms"); } } </code></pre> <p>This runs in approximately 6 ms, if I replace <code>new double[64]</code> with <code>new double[65]</code> it takes approximately 7 seconds. This problem becomes exponentially more severe if the job is spread across more and more threads, which is where my problem originates from.</p> <p>This problem also occurs with different types of arrays such as <code>int[65]</code> or <code>String[65]</code>. This problem does not occur with large strings: <code>String test = "many characters";</code>, but does start occurring when this is changed into <code>String test = i + "";</code></p> <p>I was wondering why this is the case and if it is possible to circumvent this problem.</p>
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    1. COhttp://en.wikipedia.org/wiki/CPU_cache - 64 elements will cause less cache invalidations than 65 elements (due to cache line sizes).
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    2. COOff-note: `System.nanoTime()` should be preferred over `System.currentTimeMillis()` for benchmarking.
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    3. CO@Oded: 64 doubles will not fit in a cache line. Besides which, what needs to be cached in the OP's code?
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