What is Java's -XX:+UseMembar parameter

java concurrency jvm jvm-arguments
David Rabinowitz picture David Rabinowitz · Jul 13, 2009 · Viewed 8.6k times · Source

I see this parameter in all kinds of places (forums, etc.) and the common answer it help highly concurrent servers. Still, I cannot find an official documentation from sun explaining what it does. Also, was it added in Java 6 or did it exist in Java 5?

(BTW, a good place for many hotspot VM parameters is this page)

Update: Java 5 does not boot with this parameter.

Answer

butterchicken picture butterchicken · Jul 13, 2009

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.

The use of -XX:+UseMembar 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.

The bug was introduced in 1.5, and I believe the flag exists in 1.5 and 1.6.

I've google-fu'ed this from a variety of mailing lists and JVM bugs:

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