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POInterpretation of "program order rule" in Java concurrency
 

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  1. POInterpretation of "program order rule" in Java concurrency
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    <p>Program order rule states "Each action in a thread happens-before every action in that thread that comes later in the program order"</p> <p>1.I read in another <a href="https://stackoverflow.com/questions/11353274/what-is-the-reasoning-behind-volatile-semantics-in-java-and-c-sharp">thread</a> that an <strong>action</strong> is</p> <ul> <li>reads and writes to variables</li> <li>locks and unlocks of monitors</li> <li>starting and joining with threads </li> </ul> <p>Does this mean that reads and writes can be changed in order, but reads and writes cannot change order with actions specified in 2nd or 3rd lines?</p> <p>2.What does "program order" mean? </p> <p>Explanation with an examples would be really helpful.</p> <p><strong>Additional related question</strong></p> <p>Suppose I have the following code:</p> <pre><code>long tick = System.nanoTime(); //Line1: Note the time //Block1: some code whose time I wish to measure goes here long tock = System.nanoTime(); //Line2: Note the time </code></pre> <p>Firstly, it's a single threaded application to keep things simple. Compiler notices that it needs to check the time twice and also notices a block of code that has no dependency with surrounding time-noting lines, so it sees a potential to reorganize the code, which could result in Block1 not being surrounded by the timing calls during actual execution (for instance, consider this order Line1->Line2->Block1). But, I as a programmer can see the dependency between Line1,2 and Block1. Line1 should immediately precede Block1, Block1 takes a finite amount of time to complete, and immediately succeeded by Line2.</p> <p>So my question is: Am I measuring the block correctly? </p> <ul> <li>If yes, what is preventing the compiler from rearranging the order.</li> <li>If no, (which is think is correct after going through Enno's answer) what can I do to prevent it.</li> </ul> <p>P.S.: I stole this code from <a href="https://stackoverflow.com/questions/15607265/measuring-time-interval-and-out-of-order-execution">another question</a> I asked in SO recently.</p>
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    1. USAbhishek S
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    1. COWhy don't you hit the [Java Language Specification?](http://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4.2)
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      1. USMarko Topolnik
    2. COI highly recommend picking up a copy of Java Concurrency in Practice by Goetz et al. Although it's starting to show its age a little it is still something of a gold standard for concurrent programming references. You may also find this GoogleTalk by Jeremy Manson (who co-authored the memory model itself) to be helpful: http://www.youtube.com/watch?v=WTVooKLLVT8
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      1. POInterpretation of "program order rule" in Java concurrency
      1. USAndrew Bissell
    3. CO@Andrew: Actually, I did see Jeremy Manson's video and that's why I was curious to learn more about the memory model. Looking at the example in his blog [here](http://jeremymanson.blogspot.com/2007/05/roach-motels-and-java-memory-model.html) it appears to me that his example actually violates the program order rule as he moves around the variables x and y around other monitor locks and variable accesses. Why isn't this rule being violated? If he's right, what am I missing? He also briefly talks about the roach-motel concept in his video, so I'm sure he's right.
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      1. USAbhishek S
 

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