11 out of the 17 defined PerfCounters in objectMonitor.cpp are unused in the sense that they're never incremented or otherwise manipulated after being allocated:

_sync_EmptyNotifications
_sync_PrivateA
_sync_PrivateB
_sync_SlowExit
_sync_SlowEnter
_sync_SlowNotify
_sync_SlowNotifyAll
_sync_FailedSpins
_sync_SuccessfulSpins
_sync_MonInCirculation
_sync_MonScavenged

Since these might be published as a counter via JMX, investigate if some of them should be re-implemented, otherwise remove them