RFR: 8277397: ZGC: Add JFR event for temporary latency measurements

[Stefan Karlsson]

On Thu, 18 Nov 2021 23:59:45 GMT, Jaroslav Bachorik <jbachorik at openjdk.org> wrote:

>> I often measure latencies and stalls using JFR events. I'd like to add an event that can be used for these ad-hoc measurements during development and debugging.
>
> src/hotspot/share/jfr/metadata/metadata.xml line 1037:
> 
>> 1035:   </Event>
>> 1036: 
>> 1037:   <Event name="ZThreadEvent" category="Java Virtual Machine, GC, Detailed" label="ZGC Thread Event" thread="true" experimental="true">
> 
> Can you add a description to the event?
> For anyone coming from outside it would really help to understand what this event is supposed to represent.

Great idea.

> src/jdk.jfr/share/conf/jfr/default.jfc line 778:
> 
>> 776:     </event>
>> 777: 
>> 778:     <event name="jdk.ZThreadEvent">
> 
> This is just a question - do we want/need to have this event enabled for the 'low overhead profiling' setup?

Yes. I always use default.jfc. The profile.jfc is creating too many allocation related events to make it usable when running with ZGC. I know that there has been some plans to fix that, but I don't know if that made it into JDK 17 or not.

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PR: https://git.openjdk.java.net/jdk/pull/6454