RFR: 8277397: ZGC: Add JFR event for temporary latency measurements [v3]

[Stefan Karlsson]

On Fri, 19 Nov 2021 08:49:14 GMT, Stefan Karlsson <stefank 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.
>
> Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Rename ZThreadEvent to ZThreadDebug

> 

The overhead should be virtually zero, since no event is sent. Having this turned off by default makes this feature more annoying to use. Is this causing problems for the users?

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