Question on ParallelGCThreads

[Sujit.Das at cognizant.com]

Hi All,
 
We use CMS collector for old generation collection (option -XX:+UseConcMarkSweepGC) and parallel copying collector for young generation (option -XX:+ UseParNewGC). We use ParallelGCThreads command line option (-XX:ParallelGCThreads=<desired number>) to control number of garbage collector threads.
 
My question is:
1. Is ParallelGCThreads option applicable for only minor GC or is it applicable for old generation GC also?
 
2. Since CMS collector is a non-compacting collector and if application faces memory fragmentation issue then reducing # of ParallelGCThreads is an option to reduce fragmentation. Please confirm the understanding. This is based on my reading that each garbage collection thread reserves a part of the old generation for promotions and the division of the available space into these "promotion buffers" can cause a fragmentation effect. Reducing the number of garbage collector threads will reduce this fragmentation effect as will increasing the size of the old generation. 
 
Thanks,
Sujit
 


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