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POPerformance Explanation: code runs slower after warm up
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12035676
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12041431
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26
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2012-08-20T10:01:40.280
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2018-03-21T14:12:59.270
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2012-08-20T13:44:54.420
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text
Body
The code below runs the exact same calculation 3 times (it does not do much: basically adding all numbers from 1 to 100m). The first 2 blocks run approximately 10 times faster than the third one. I have run this test program more than 10 times and the results show very little variance. If anything, I would expect the third block to run faster (JIT compilation), but the typical output is: <blockquote> 35974537 36368455 296471550 </blockquote> Can somebody explain what is happening? (Just to be clear, I'm not trying to fix anything here, just trying to better understand what is going on) Note: <ul> <li>no GC is run during the program (monitored with <code>-XX:+PrintGC</code>)</li> <li>tested with Oracle JDK versions 1.6.0_30, 1.7.0_02 and 1.7.0_05</li> <li>also tested with the following parameters: <code>-XX:+PrintGC -Xms1000m -Xmx1000m -XX:NewSize=900m</code> => same result</li> <li>it the block is put in a loop instead, all runs are fast</li> <li>if the block is extracted to a method, all runs are fast (whether the method is called 3 times or in a loop makes no difference)</li> </ul> <pre><code>public static void main(String... args) { //three identical blocks { long start = System.nanoTime(); CountByOne c = new CountByOne(); int sum = 0; for (int i = 0; i < 100000000; i++) { sum += c.getNext(); } if (sum != c.getSum()) throw new IllegalStateException(); //use sum long end = System.nanoTime(); System.out.println((end - start)); } { long start = System.nanoTime(); CountByOne c = new CountByOne(); int sum = 0; for (int i = 0; i < 100000000; i++) { sum += c.getNext(); } if (sum != c.getSum()) throw new IllegalStateException(); //use sum long end = System.nanoTime(); System.out.println((end - start)); } { long start = System.nanoTime(); CountByOne c = new CountByOne(); int sum = 0; for (int i = 0; i < 100000000; i++) { sum += c.getNext(); } if (sum != c.getSum()) throw new IllegalStateException(); //use sum long end = System.nanoTime(); System.out.println((end - start)); } } public static class CountByOne { private int i = 0; private int sum = 0; public int getSum() { return sum; } public int getNext() { i += 1; sum += i; return i; } } </code></pre>
Tags
<java><performance><jvm><jit>
Title
Performance Explanation: code runs slower after warm up
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