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    <p>Your experiment is broken, not your laptop. See here for a discussion and some tools to help measure performance: <a href="https://stackoverflow.com/questions/1237181/java-performance-timing-library">Java performance timing library</a></p> <p>Below are some results that contract yours. Also I modified your code to be more rigorous and careful in how it takes measurements.</p> <hr> <p>My environment is Linux (Mint 14 which is based on Ubuntu 12.10) using the Sun JDK 1.6.0_38</p> <p>With 1.5G of heap for the big example, ie -Xmx1512</p> <hr> <p>Note: interesting. Could be my result is different because the array size is different below. Will re-run and update. </p> <p>Nope: the result is similar in there is not much difference in the mean. But what is interesting is the difference against the short run, ie 21092.5 (/10 = 2109.2) vs 1645.2 which may be slower because of memory paging.</p> <p>result with <code>static long[] arr = new long[100000000];</code> (the original array size in question)</p> <blockquote> <p>Write: DescriptiveStatistics: n: 10 min: 20893.0 max: 22190.0 mean: 21092.5 std dev: 390.90727800848117 median: 20953.5 skewness: 3.0092198852491543 kurtosis: 9.264808973899097</p> <p>Read: DescriptiveStatistics: n: 10 min: 21668.0 max: 22736.0 mean: 21892.5 std dev: 318.31509546359877 median: 21766.5 skewness: 2.5034216544466124 kurtosis: 6.560838306717343</p> </blockquote> <hr> <p>I'm not seeing a huge difference in reads vs writes. I changed the experiment to measure 10 times on a slightly smaller array (result is the same number of reads/writes). Feel free to re-run with a larger size array or sample size.</p> <blockquote> <p>Write: DescriptiveStatistics: n: 10 min: 1584.0 max: 1799.0 mean: 1645.2 std dev: 59.51619760853156 median: 1634.5 skewness: 2.137918517160786 kurtosis: 5.764166551997385</p> <p>Read: DescriptiveStatistics: n: 10 min: 1568.0 max: 2202.0 mean: 1689.0 std dev: 186.93908693000031 median: 1623.0 skewness: 2.770215113912315 kurtosis: 8.12245132320571</p> </blockquote> <p>Here is a modified version of your code that does more samples:</p> <pre><code>import java.util.Random; import org.apache.commons.lang.time.StopWatch; import org.apache.commons.math.stat.descriptive.DescriptiveStatistics; public class Test { static Random random = new Random(); // static long[] arr = new long[100000000]; static long[] arr = new long[10000000]; static void seqReadRandWrite() { for (int i = 0; i &lt; arr.length; i++) { int at = Math.abs(random.nextInt()) % arr.length; arr[at] = arr[i]; } } static void seqWriteRandRead() { for (int i = 0; i &lt; arr.length; i++) { int at = Math.abs(random.nextInt()) % arr.length; arr[i] = arr[at]; } } public static void main(String[] args) throws Exception { StopWatch timer = new StopWatch(); int count = 10; // warm up for (int i=0; i&lt;3; i++){ seqReadRandWrite(); } DescriptiveStatistics write = new DescriptiveStatistics(); for (int i=0; i&lt;count; i++){ timer.reset(); timer.start(); seqReadRandWrite(); timer.stop(); write.addValue(timer.getTime()); } System.out.println("Write: " + write); // warm up for (int i=0; i&lt;3; i++){ seqWriteRandRead(); } DescriptiveStatistics read = new DescriptiveStatistics(); for (int i=0; i&lt;count; i++){ timer.reset(); timer.start(); seqWriteRandRead(); timer.stop(); read.addValue(timer.getTime()); } System.out.println("Read: " + read); } } </code></pre>
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    1. POJava VM memory performance - Are Array writes faster than Array reads?
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    1. COOn my laptop, your program (without any changes) measures that writes are more than twice as fast as reads. Maybe everybody's laptop is broken but yours :)
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    2. COTom: What is your environment? Can you please try the test in my answer?
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    3. COInteresting - what OS are you using?
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