Summary

Deprecate the -XX:MonitorBound=N flag

Problem

The product flag, MonitorBound, was added in JDK 7-B99 by JDK-6852873. It was also backported to various JDK6 update releases. The flag was added as a way to control how often the system invoked a cleanup safepoint for reducing the number of idle ObjectMonitors.

An experimental flag MonitorUsedDeflationThreshold was added in JDK 10-B21 by JDK-8181859 to control when idle monitor deflation is invoked. The new flag is described:

  experimental(intx, MonitorUsedDeflationThreshold, 90, \
                "Percentage of used monitors before triggering cleanup " \
                "safepoint which deflates monitors (0 is off). " \
                "The check is performed on GuaranteedSafepointInterval.") \
                range(0, 100)

A policy based on the percentage of used monitors is a more flexible mechanism that the fixed limit specified by MonitorBound. The MonitorBound option should have been deprecated back in JDK 10, but we missed that detail.

Solution

Deprecate the -XX:MonitorBound=N flag.

The flag will still be accepted and acted upon but a deprecation warning will be issued.

The flag will be obsoleted in JDK 15 and removed in JDK 16.

Specification

diff -r f63f50a4bf43 src/hotspot/share/runtime/arguments.cpp
--- a/src/hotspot/share/runtime/arguments.cpp   Thu Sep 12 11:07:35 2019 -0700

+++ b/src/hotspot/share/runtime/arguments.cpp   Thu Sep 12 20:44:45 2019 -0400
@@ -539,6 +539,7 @@
   { "FlightRecorder",               JDK_Version::jdk(13), JDK_Version::undefined(), JDK_Version::undefined() },
   { "FieldsAllocationStyle",        JDK_Version::jdk(14), JDK_Version::jdk(15), JDK_Version::jdk(16) },
   { "CompactFields",                JDK_Version::jdk(14), JDK_Version::jdk(15), JDK_Version::jdk(16) },
+  { "MonitorBound",                 JDK_Version::jdk(14), JDK_Version::jdk(15), JDK_Version::jdk(16) },

   // --- Deprecated alias flags (see also aliased_jvm_flags) - sorted by obsolete_in then expired_in:
   { "DefaultMaxRAMFraction",        JDK_Version::jdk(8),  JDK_Version::undefined(), JDK_Version::undefined() },
diff -r f63f50a4bf43 src/hotspot/share/runtime/globals.hpp
--- a/src/hotspot/share/runtime/globals.hpp Thu Sep 12 11:07:35 2019 -0700
+++ b/src/hotspot/share/runtime/globals.hpp Thu Sep 12 20:44:45 2019 -0400
@@ -713,7 +713,7 @@
           "Use LWP-based instead of libthread-based synchronization "       \
           "(SPARC only)")                                                   \
                                                                             \
-  product(intx, MonitorBound, 0, "Bound Monitor population")                \
+  product(intx, MonitorBound, 0, "(Deprecated) Bound Monitor population")   \
           range(0, max_jint)                                                \
                                                                             \
   experimental(intx, MonitorUsedDeflationThreshold, 90,                     \