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I see following settings in production for our tomcat

-XX:+UseConcMarkSweepGC -XX:+UseParNewGC

What does it mean to have two collectors? Is the UseConcMarkSweepGC for older gen and ParNewGC for new gen? or is it that UseParNewGC is the only one that would run?

Looking at java specs, you can use -XX:+UseParallelGC in conjuction with ConcMarkSweepk, but nothing is mentioned about UseParNewGC

Jimm
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  • Explained at http://stackoverflow.com/questions/2101518/difference-between-xxuseparallelgc-and-xxuseparnewgc – jdiver May 02 '14 at 15:56

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According to this document, you can combine the two, since they control different aspects of the garbage collection:

-XX:+UseParNewGC—Uses a parallel version of the young generation copying collector alongside the default collector. This minimizes pauses by using all available CPUs in parallel. The collector is compatible with both the default collector and the Concurrent Mark and Sweep (CMS) collector.

Keppil
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