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    <p>In order to optimise performance, the JVM uses a "pseudo memory barrier" in code to act as a fencing instruction when synchronizing across multiple processors. It is possible to revert back to a "true" memory barrier instruction, but this can have a noticeable (and bad) effect upon performance.</p> <p>The use of <code>-XX:+UseMembar</code> causes the VM to revert back to true memory barrier instructions. This parameter was originally intended to exist temporarily as a verification mechanism of the new pseudo-barrier logic, but it turned out that the new pseudo-memory barrier code introduced some synchronization issues. I believe these are now fixed, but until they were, the acceptable way to get around these issues was to use the reinstated flag.</p> <p>The bug was introduced in 1.5, and I believe the flag exists in 1.5 and 1.6.</p> <p>I've google-fu'ed this from a variety of mailing lists and JVM bugs:</p> <ul> <li><a href="http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6546278" rel="nofollow noreferrer">http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6546278</a></li> <li><a href="http://en.wikipedia.org/wiki/Memory_barrier" rel="nofollow noreferrer">The obligatory Wikipedia link to memory barriers (fencing instructions)</a></li> <li><a href="http://blogs.msdn.com/oldnewthing/archive/2004/05/28/143769.aspx" rel="nofollow noreferrer">A classic post from Raymond Chen</a></li> <li><a href="https://stackoverflow.com/questions/286629/what-is-a-memory-fence">SO question on fencing</a></li> </ul>
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    1. POWhat is Java's -XX:+UseMembar parameter
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    1. COWhat I don't know (and would love to) is how the pseudo barrier code works...
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    2. COAlso see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6822370 : on newer cpus and VMs with this bug you may have very odd synchronization issues (fixed in 6u18)
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