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RFE: 8153224 Monitor deflation prolong safepoints
         https://bugs.openjdk.java.net/browse/JDK-8153224

Full Webrev: http://cr.openjdk.java.net/~dcubed/8153224-webrev/67-for-jdk13.full/

Inc Webrev: http://cr.openjdk.java.net/~dcubed/8153224-webrev/67-for-jdk13.inc/

Background

This patch for Async Monitor Deflation is based on Carsten Varming's

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   ((gMonitorPopulation - gMonitorFreeCount) / gMonitorPopulation) > NN%

• • When async deflation is enabled, MonitorUsedDeflationThreshold is checked for the global in-use list and for each per-thread in-use list. (Coming in CR8.)

• If MonitorBound is exceeded (default is 0 which means off), cleanup safepoint will be induced.
• For this option, exceeded means:

(gMonitorPopulation - gMonitorFreeCount) > MonitorBound

• When async deflation is enabled, MonitorBound is checked for the global in-use list and for each per-thread in-use list so a smaller, scalable value can be used. (Coming in CR8.)This is a very difficult option to use correctly as it does not scale.

• Changes to the safepoint deflation mechanism by the Async Monitor Deflation project (when async deflation is enabled):
  • If System.gc() is called, then a special deflation request is made which invokes the safepoint deflation mechanism.
  • Added the AsyncDeflationInterval diagnostic option (default 250 millis, 0 means off) to prevent MonitorUsedDeflationThreshold requests from swamping the ServiceThread.
    • Description: Async deflate idle monitors every so many milliseconds when MonitorUsedDeflationThreshold is exceeded (0 is off).
    • A special deflation request can cause an async deflation to happen sooner than AsyncDeflationInterval.
  • SafepointSynchronize::do_cleanup_tasks() now calls:
    • ObjectSynchronizer::is_safepoint_deflation_needed() instead of ObjectSynchronizer::is_cleanup_needed().
    • is_safepoint_deflation_needed() returns true only if a special deflation request is made (see above).
  • ObjectSynchronizer::do_safepoint_work() only does the safepoint cleanup tasks if there is a special deflation request. Otherwise it just sets the is_async_deflation_requested flag and notifies the ServiceThread.
  • ObjectSynchronizer::deflate_idle_monitors() and ObjectSynchronizer::deflate_thread_local_monitors() do nothing unless there is a special deflation request.

• Changes to the ServiceThread mechanism by the Async Monitor Deflation project (when async deflation is enabled):

  • The ServiceThread will wake up every GuaranteedSafepointInterval to check for cleanup tasks.

    • This allows is_async_deflation_needed() to be checked at the same interval.

  • The ServiceThread handles deflating global idle monitors and sets the omShouldDeflateIdleMonitors request flag for the per-thread idle monitor check.

• Other invocation changes by the Async Monitor Deflation project (when async deflation is enabled):

  • VM_Exit::doit_prologue() will request a special cleanup to reduce the noise in 'monitorinflation' logging at VM exit time.

  • Before the final safepoint in a non-System.exit() end to the VM, we will request a special cleanup to reduce the noise in 'monitorinflation' logging at VM exit time.

Gory Details

• Counterpart function mapping for those that know the existing code:
  • ObjectSynchronizer class:
    • deflate_idle_monitors() has deflate_global_idle_monitors_using_JT(), deflate_per_thread_idle_monitors_using_JT(), and deflate_common_idle_monitors_using_JT().
    • deflate_monitor_list() has deflate_monitor_list_using_JT()
    • deflate_monitor() has deflate_monitor_using_JT()
  • ObjectMonitor class:
    • is_busy() has is_busy_async()
    • clear() has clear_using_JT()
• These functions recognize the Async Monitor Deflation protocol and adapt their operations:
  
  • ObjectMonitor::EnterI()
  • ObjectMonitor::ReenterI()
• Also these functions had to adapt and retry their operations:
  
  • ObjectSynchronizer::quick_enter()
  • ObjectSynchronizer::FastHashCode()
  • ObjectSynchronizer::current_thread_holds_lock()
  • ObjectSynchronizer::query_lock_ownership()
  • ObjectSynchronizer::get_lock_owner()
  • ObjectSynchronizer::monitors_iterate()
  • ObjectSynchronizer::inflate_helper()
  • ObjectSynchronizer::inflate() 
• Various assertions had to be modified to pass without their real check when AsyncDeflateIdleMonitors is true; this is due to the change in semantics for the ObjectMonitor owner field.
• ObjectMonitor has a new allocation_state field that supports three states: 'Free', 'New', 'Old'. Async Monitor Deflation is only applied to ObjectMonitors that have reached the 'Old' state.
  • Note: Prior to CR1/v2.01/4-for-jdk13, the allocation state was transitioned from 'New' to 'Old' in deflate_monitor_via_JT(). This meant that deflate_monitor_via_JT() had to see an ObjectMonitor twice before deflating it. This policy was intended to prevent oscillation from 'New' → 'Old' and back again.
  • In CR1/v2.01/4-for-jdk13, the allocation state is transitioned from 'New' -> "Old" in inflate(). This makes ObjectMonitors available for deflation earlier. So far there has been no signs of oscillation from 'New' → 'Old' and back again.
• ObjectMonitor has a new ref_count field that is used as part of the async deflation protocol and to indicate that an ObjectMonitor* is in use so the ObjectMonitor should not be deflated; this is needed for operations on non-busy monitors so that ObjectMonitor values don't change while they are being queried. There is a new ObjectMonitorHandle helper to manage the ref_count.
• The ObjectMonitor::owner() accessor detects DEFLATER_MARKER and returns NULL in that case to minimize the places that need to understand the new DEFLATER_MARKER value.
• System.gc()/JVM_GC() causes a special monitor list cleanup request which uses the safepoint based monitor list mechanism. So even if AsyncDeflateIdleMonitors is enabled, the safepoint based mechanism is still used by this special case.
  • This is necessary for those tests that do something to cause an object's monitor to be inflated, clear the only reference to the object and then expect that enough System.gc() calls will eventually cause the object to be GC'ed even when the thread never inflates another object's monitor. Yes, we have several tests like that. :-)