Summary

Remove ObjectMonitor performance counters, exposed in sun.rt._sync* namespace.

Problem

There is a bunch of ObjectMonitor performance counters:

PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = nullptr;
PerfCounter * ObjectMonitor::_sync_FutileWakeups = nullptr;
PerfCounter * ObjectMonitor::_sync_Parks = nullptr;
PerfCounter * ObjectMonitor::_sync_Notifications = nullptr;
PerfCounter * ObjectMonitor::_sync_Inflations = nullptr;
PerfCounter * ObjectMonitor::_sync_Deflations = nullptr;
PerfLongVariable * ObjectMonitor::_sync_MonExtant = nullptr; 

They seem to be seldom used, and their continued maintenance comes with at least two problems, both of which come down to intrinsic race against VM shutdown, see JDK-8049304 and JDK-8348402. Additionally, these get updated on locking paths, and attempts to do extra things, including synchronization, might obscure some of the bugs.

Some of these counters are covered by related JFR events:

• _sync_ContendedLockAttempts: covered by existing JavaMonitorEnter event
• _sync_FutileWakeups: not covered, we think this is unnecessary to track
• _sync_Parks: covered by existing JavaMonitorWait event
• _sync_Notifications: covered by new JavaMonitorNotify event (JDK-8351187)
• _sync_Inflations: covered by existing JavaMonitorInflate event
• _sync_Deflations: covered by new JavaMonitorDeflate event (JDK-8351142)
• _sync_MonExtant: covered by new JavaMonitorStatistics event (JDK-8351142)

These counters are also accessible through internal APIs or jcmd:

% jcmd 8090 PerfCounter.print | grep _sync
sun.rt._sync_ContendedLockAttempts=10045275
sun.rt._sync_Deflations=222972
sun.rt._sync_FutileWakeups=4184363
sun.rt._sync_Inflations=222991
sun.rt._sync_MonExtant=7
sun.rt._sync_Notifications=1521211
sun.rt._sync_Parks=7667155

Solution

The solution is to remove these counters.

Specification

All counters in sun.rt._sync* would be gone, see related PR: https://github.com/openjdk/jdk/pull/23326