RFR: 8213966: The ZGC JFR events should be marked as experimental

[Erik Gahlin]

Hi,

Could I have a review of a fix that will mark the ZGC events for JFR as 
experimental.

The experimental attribute was created so that JVM developers could add 
events more freely to HotSpot without the burden to write unit tests or 
commit to a particular event design. Experimental events are not shown 
in JMC by default, but can be made visible in the Preference dialog. See 
the following picture.

http://cr.openjdk.java.net/~egahlin/8213966/experimental.png

As part of this change, I also moved the Z Page Allocation event into 
JVM / GC / Detailed category with the other ZGC events. It seemed more 
appropriate, as the Java category is meant for events that are more 
close to the Java application when doing troubleshooting.

The experimental=true attribute can be removed/changed in a future 
release. At that time, it may make sense to revisit the event layout. 
Perhaps break up the Z Statistics event into more fine-grained events 
with proper labels and descriptions. Marking the ZGC events as 
experimental also means the events won't require special treatment in 
TestLookForUntestedEvents test.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8213966

Webrev:
http://cr.openjdk.java.net/~egahlin/8213966/

Thanks
Erik