From Brice Dutheil:

Motivation

When turning the GC flag knobs, it is useful to be able to quickly see
the GC configuration, however the GC configuration page only show 3
events, if one wants to have a look at flags he has to either check
the command line or inspect the JVM internal page to find relevant
GC flags. Therefore having these flags right there is useful to get
a quick picture of the current configuration.

Current limitations

At this time the selected flags depends on the GC algorithm which is
derived from the name of the old collector name. This is simple,
however it prevents to identify EpsilonGC because whose old
collector name is SerialOld, which is the same as SerialGC.

There are some global flags in gc/shared/gc_globals.hpp that
I didn't include as I didn't use most of them. I'm not sure these
flags are seen much in the wild.

Remarks

As mentionned above the use of the OldCollector name may not be the
best choice, I would like to refactor this part to use the Use*GC
flags instead. I am a bit unsure at this time how to use the
existing API.