RFR: 8003216: Add JFR event indicating explicit System.gc() cal

[Stefan Johansson]

Hi Erik,

> 26 feb. 2020 kl. 13:56 skrev Per Liden <per.liden at oracle.com>:
> 
> Hi Erik,
> 
> On 2020-02-26 13:50, Erik Gahlin wrote:
>> Hi,
>> Could I have a review of a JFR event that is emitted when System.gc() is called.
>> Purpose is to collect the stack trace. It is not sufficient with the cause field that the GarbageCollection event has today.
>> Bug:
>> https://bugs.openjdk.java.net/browse/JDK-8003216
>> Webrev:
>> http://cr.openjdk.java.net/~egahlin/8003216/
> 
> 489     EventSystemGC event;
> 490     event.commit();
> 491     Universe::heap()->collect(GCCause::_java_lang_system_gc);
> 
> Don't you want the commit() call after the call to collect(), to get the timing right?

I was thinking the same thing, could also be nice to have the GC-id associated with the event to make it easy to match it to GC-logs and other GC-events. Not sure how to easily get the GC-id though, since it’s not set at the time we commit the event.

I guess if the event has the correct span with timestamps it will be easy to figure out which other events are associated with it, even without the GC-id.

Cheers,
Stefan

> 
> cheers,
> Per
> 
>> Testing:
>> tier1+tier2+jdk/jdk/jfr
>> Thanks
>> Erik