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POUnderstanding the output of -XX:+PrintCompilation
 

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  1. POUnderstanding the output of -XX:+PrintCompilation
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    <p>I am running some micro benchmarks on Java list iteration code. I have used -XX:+PrintCompilation, and -verbose:gc flags to ensure that nothing is happening in the background when the timing is being run. However, I see something in the output which I cannot understand.</p> <p><strong>Here's the code, I am running the benchmark on:</strong></p> <pre><code>import java.util.ArrayList; import java.util.List; public class PerformantIteration { private static int theSum = 0; public static void main(String[] args) { System.out.println("Starting microbenchmark on iterating over collections with a call to size() in each iteration"); List&lt;Integer&gt; nums = new ArrayList&lt;Integer&gt;(); for(int i=0; i&lt;50000; i++) { nums.add(i); } System.out.println("Warming up ..."); //warmup... make sure all JIT comliling is done before the actual benchmarking starts for(int i=0; i&lt;10; i++) { iterateWithConstantSize(nums); iterateWithDynamicSize(nums); } //actual System.out.println("Starting the actual test"); long constantSizeBenchmark = iterateWithConstantSize(nums); long dynamicSizeBenchmark = iterateWithDynamicSize(nums); System.out.println("Test completed... printing results"); System.out.println("constantSizeBenchmark : " + constantSizeBenchmark); System.out.println("dynamicSizeBenchmark : " + dynamicSizeBenchmark); System.out.println("dynamicSizeBenchmark/constantSizeBenchmark : " + ((double)dynamicSizeBenchmark/(double)constantSizeBenchmark)); } private static long iterateWithDynamicSize(List&lt;Integer&gt; nums) { int sum=0; long start = System.nanoTime(); for(int i=0; i&lt;nums.size(); i++) { // appear to do something useful sum += nums.get(i); } long end = System.nanoTime(); setSum(sum); return end-start; } private static long iterateWithConstantSize(List&lt;Integer&gt; nums) { int count = nums.size(); int sum=0; long start = System.nanoTime(); for(int i=0; i&lt;count; i++) { // appear to do something useful sum += nums.get(i); } long end = System.nanoTime(); setSum(sum); return end-start; } // invocations to this method simply exist to fool the VM into thinking that we are doing something useful in the loop private static void setSum(int sum) { theSum = sum; } } </code></pre> <p><br/> <strong>Here's the output.</strong></p> <pre><code> 152 1 java.lang.String::charAt (33 bytes) 160 2 java.lang.String::indexOf (151 bytes) 165 3Starting microbenchmark on iterating over collections with a call to size() in each iteration java.lang.String::hashCode (60 bytes) 171 4 sun.nio.cs.UTF_8$Encoder::encodeArrayLoop (490 bytes) 183 5 java.lang.String::lastIndexOf (156 bytes) 197 6 java.io.UnixFileSystem::normalize (75 bytes) 200 7 java.lang.Object::&lt;init&gt; (1 bytes) 205 8 java.lang.Number::&lt;init&gt; (5 bytes) 206 9 java.lang.Integer::&lt;init&gt; (10 bytes) 211 10 java.util.ArrayList::add (29 bytes) 211 11 java.util.ArrayList::ensureCapacity (58 bytes) 217 12 java.lang.Integer::valueOf (35 bytes) 221 1% performance.api.PerformantIteration::main @ 21 (173 bytes) Warming up ... 252 13 java.util.ArrayList::get (11 bytes) 252 14 java.util.ArrayList::rangeCheck (22 bytes) 253 15 java.util.ArrayList::elementData (7 bytes) 260 2% performance.api.PerformantIteration::iterateWithConstantSize @ 19 (59 bytes) 268 3% performance.api.PerformantIteration::iterateWithDynamicSize @ 12 (57 bytes) 272 16 performance.api.PerformantIteration::iterateWithConstantSize (59 bytes) 278 17 performance.api.PerformantIteration::iterateWithDynamicSize (57 bytes) Starting the actual test Test completed... printing results constantSizeBenchmark : 301688 dynamicSizeBenchmark : 782602 dynamicSizeBenchmark/constantSizeBenchmark : 2.5940773249184588 </code></pre> <p><br/> <strong>I don't understand these four lines from the output.</strong></p> <pre><code>260 2% performance.api.PerformantIteration::iterateWithConstantSize @ 19 (59 bytes) 268 3% performance.api.PerformantIteration::iterateWithDynamicSize @ 12 (57 bytes) 272 16 performance.api.PerformantIteration::iterateWithConstantSize (59 bytes) 278 17 performance.api.PerformantIteration::iterateWithDynamicSize (57 bytes) </code></pre> <p><br/></p> <ul> <li>Why are both these methods being compiled twice ?</li> <li>How do I read this output... what do the various numbers mean ?</li> </ul>
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    1. USThomas Jungblut
    1. USParag
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    1. COThanks @Thomas Jungblut Not sure how I missed out -XX:+PrintCompilation from the title
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      1. USParag
    2. COnp, here is a good resource about compilations: https://gist.github.com/1165804#file_notes.md
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      1. USThomas Jungblut
 

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