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POStrange concurrency behaviour on Java 6 Mac OS X Lion
 

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  1. POStrange concurrency behaviour on Java 6 Mac OS X Lion
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    <p>I've been experimenting with difference between intrinsic locks and <code>java.util.concurrent.ReentrantLock</code> for some time now. I've found very strange thing. Consider following code:</p> <pre><code>public class WriteOnceRunAnywhere { private static long counter = 0; public static void main(String[] args) throws InterruptedException { final int numThreads = 2; final int numIterations = Integer.MAX_VALUE; Runnable inc = new Runnable() { public void run() { for (int i = 0; i &lt; numIterations; i++) { increment(); if (i % 10000000 == 0) System.out.println(Thread.currentThread().getName()); } } }; for (int i = 0; i &lt; numThreads; i++) new Thread(inc).start(); } public static synchronized void increment() { counter++; } } </code></pre> <p>Simple thing, no fancy stuff. Right? Happens to break! Most likely when you run it, it won't end. After some ping-pong between threads you'll see only one thread is actually running. The other <em>hangs</em> forever:</p> <blockquote> <p>Thread-1 Thread-2 Thread-1 Thread-2 Thread-1 ... Thread-2 Thread-2 Thread-2 Thread-2 Thread-2 Thread-2 Thread-2 Thread-2 Thread-2 ...</p> </blockquote> <p>After that, java process can't accept <code>jvisualvm</code> connection. CPU load drops and continuously stays at about 1.0%. </p> <blockquote> <p>Mac OS X Lion 10.7.2 (11C74), 2.53 GHz Intel Core i5</p> <p>java version "1.6.0_29" Java(TM) SE Runtime Environment (build 1.6.0_29-b11-402-11M3527) Java HotSpot(TM) 64-Bit Server VM (build 20.4-b02-402, mixed mode)</p> </blockquote> <p>Can someone tell me what the heck is happening here?</p> <p><strong>UPD</strong> Looks like the bug will be fixed in 1.6.30 <a href="https://stackoverflow.com/a/9088122/381105">see</a></p>
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