Thank Wolf-Dieter Fink for your reply. However, I can't understand what do you mean by conflicting GC?
What is the difference between ParallelGC and ParallelOldGc?? In the link you proposed it is mentiod that ParallelOldGc automatically enable ParallelGC.
I am always measuring what's happening by viewing GC logs and HeapDumps.
The JVM version is build 1.5.0_12-b04. :)