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    <p>Try turning off <code>-XX:-UseBiasedLocking</code> It probably ping-pongs until the code is optimised (try <code>-XX:+PrintCompilation</code> and then it effectively determines the program will run faster if only one thread holds the lock. e.g. the synchronized is effectively moved outside the loop.</p> <p>You will see this behaviour often when you using multiple threads to do something which is faster to do single threaded.</p> <hr> <p>You can either have each thread alternatively increment the value or you can have one loop run until it stops (at Integer.MAX_VALUE) and then the second one.</p> <pre><code>public class CountingWithThreads { static long count1 = 0; static long count2 = 0; public static void main(String... args) throws InterruptedException { long start1 = System.nanoTime(); Thread t1a = new Thread(new CountsSingleThreaded(Integer.MAX_VALUE)); Thread t1b = new Thread(new CountsSingleThreaded(Integer.MAX_VALUE)); t1a.start(); t1b.start(); t1a.join(); t1b.join(); long time1 = System.nanoTime() - start1; System.out.printf("Single threaded, took %.1f second to count to %,d%n", time1 / 1e9, count1); long start2 = System.nanoTime(); Thread t2a = new Thread(new CountsAlternatively(true, 1000000)); Thread t2b = new Thread(new CountsAlternatively(false, 1000000)); t2a.start(); t2b.start(); t2a.join(); t2b.join(); long time2 = System.nanoTime() - start2; System.out.printf("Alternating multi-threaded took %.1f second to count to %,d%n", time2 / 1e9, count2); } static class CountsSingleThreaded implements Runnable { private final long maxValue; CountsSingleThreaded(long maxValue) { this.maxValue = maxValue; } public void run() { synchronized (CountingWithThreads.class) { for (long i = 0; i &lt; maxValue; i++) { count1++; } } } } static class CountsAlternatively implements Runnable { private final boolean even; private final long maximum; CountsAlternatively(boolean even, long maximum) { this.even = even; this.maximum = maximum; } public void run() { try { synchronized (CountingWithThreads.class) { while (count2 &lt; maximum) if (((count2 &amp; 1) == 0) == even) { count2++; CountingWithThreads.class.notify(); } else { CountingWithThreads.class.wait(); } } } catch (InterruptedException e) { e.printStackTrace(); } } } } </code></pre> <p>prints</p> <pre><code>Single threaded, took 2.3 second to count to 4,294,967,294 Alternating multi-threaded took 3.4 second to count to 1,000,000 </code></pre> <p>Running one thread to completion one at a time is the most efficient. If you force strict alternating of the counters (the most extreme example of mult-threading) it is over 1000x slower.</p>
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    1. POStrange concurrency behaviour on Java 6 Mac OS X Lion
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    1. USPeter Lawrey
    1. USPeter Lawrey
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    1. CO@Pater Lawrey I've already tried biased locking option -- same stuff
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    2. COIn that case, I suspect the loop optimisation. See if the method is compiled just before the behaviour changes.
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      1. USPeter Lawrey
    3. COAnyway it's not right! Right? ^)
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