JMX old gen GC count is incremented by 2 at a System.gc() in CMS in JDK6u23-6u26, while it incremented by 1 in 6u22 and earlier.

command prompt

D:\sdk\groovy-1.7.6\bin>set JAVA_OPTS=-XX:+UseConcMarkSweepGC

D:\sdk\groovy-1.7.6\bin>java -version
java version "1.6.0_26"
Java(TM) SE Runtime Environment (build 1.6.0_26-b03)
Java HotSpot(TM) Client VM (build 20.1-b02, mixed mode, sharing)

D:\sdk\groovy-1.7.6\bin>groovysh
Groovy Shell (1.7.6, JVM: 1.6.0_26)
Type 'help' or '\h' for help.
-------------------------------------------------------------------------------
groovy:000> import java.lang.management.ManagementFactory
===> [import java.lang.management.ManagementFactory]
groovy:000> b = ManagementFactory.garbageCollectorMXBeans[1]
===> sun.management.GarbageCollectorImpl@b34b1
groovy:000> b.name
===> ConcurrentMarkSweep
groovy:000> b.collectionCount
===> 0
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 2
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 4
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 6
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 8
groovy:000> quit

D:\sdk\groovy-1.7.6\bin>set JAVA_HOME=\sdk\jdk1.6.0_20

D:\sdk\groovy-1.7.6\bin>groovysh
Groovy Shell (1.7.6, JVM: 1.6.0_20)
Type 'help' or '\h' for help.
-------------------------------------------------------------------------------
groovy:000> import java.lang.management.ManagementFactory
===> [import java.lang.management.ManagementFactory]
groovy:000> b = ManagementFactory.garbageCollectorMXBeans[1]
===> sun.management.GarbageCollectorImpl@1e8fa70
groovy:000> b.name
===> ConcurrentMarkSweep
groovy:000> b.collectionCount
===> 0
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 1
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 2
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 3
groovy:000> quit

D:\sdk\groovy-1.7.6\bin>set JAVA_HOME=\sdk\jdk1.6.0_23

D:\sdk\groovy-1.7.6\bin>groovysh
Groovy Shell (1.7.6, JVM: 1.6.0_23)
Type 'help' or '\h' for help.
-------------------------------------------------------------------------------
groovy:000> import java.lang.management.ManagementFactory
===> [import java.lang.management.ManagementFactory]
groovy:000> b = ManagementFactory.garbageCollectorMXBeans[1]
===> sun.management.GarbageCollectorImpl@136bdda
groovy:000> b.name
===> ConcurrentMarkSweep
groovy:000> b.collectionCount
===> 0
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 2
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 4
groovy:000> System.gc()
===> null
groovy:000> b.collectionCount
===> 6
groovy:000> quit