Monday, September 29, 2014

Java JVM Flag PrintConcurrentLocks

When all hell break loose and you have threads hanging waiting for a lock, it's PARAMOUNT to be able to determine WHO is holding that lock.

By default, the option -XX:+PrintConcurrentLocks is not enabled. if you enable it, and you do a "kill -3 PID", you should get for each thread the list of locks being held. This option "should be" safe in PROD, despite of the Oracle warnings.

However, you can get the same info using
jstack -l PID

"Reference Handler" daemon prio=10 tid=0x000000004e8b6000 nid=0x48c8 in Object.wait() [0x000000004104b000]
   java.lang.Thread.State: WAITING (on object monitor)
        at java.lang.Object.wait(Native Method)
        - waiting on <0x0000000782f36550> (a java.lang.ref.Reference$Lock)
        at java.lang.Object.wait(
        at java.lang.ref.Reference$
        - locked <0x0000000782f36550> (a java.lang.ref.Reference$Lock)

   Locked ownable synchronizers:
        - None

The "Locked ownable synchronizers:" bit is the one you would get extra by using the PrintConcurrentLocks flag.

Anyway this is only a superficial analysis... luckily working with OSB I never had to deal with locking issues... so I cannot claim to be an expert.

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