This also allows overriding complex settings as a JSON value and adds documentation about these overrides to conf/jvm-server.options
patch by Paulo Motta; reviewed by Stefan Miklosovic, David Capwell for CASSANDRA-20749
G1 can rely on `-XX:G1NewSizePercent=50` to always floor the young generation's size to 50% of the heap. (In production this can be raised to 66% for optimal performance.)
Fixing the young generation size with `-Xmn` is only applicable to CMS, so move it to the appropriate section.
When using G1, we can automatically set `-XX:ParallelGCThreads` and `-XX:ConcGCThreads` to the number of system cpu cores.
The auto-generated heap size is now half the server's physical RAM, capped at 16G for CMS and 31G for G1.
Increase MaxTenuringThreshold from 1 to 2, as evidence has shown it has no cost (over values of zero or one), but can have significant benefits in keeping objects in the young generation. While, values above 2 don't have any noticeable benefit.
Always set CASSANDRA_HEAPDUMP_DIR, default it to $CASSANDRA_LOG_DIR, to avoid hprof filling up unexpected disk volumes. Assumption here is that the logs directory is large enough to handle these dumps, and/or operators are monitoring these directories more than other randon/unknown directories.
patch by Mick Semb Wever; reviewed by Jon Haddad for CASSANDRA-20296
Josh McKenzie