RFR: 8266936: Add a finalization JFR event [v2]

[Alan Bateman]

On Wed, 19 May 2021 06:20:59 GMT, Erik Gahlin <egahlin at openjdk.org> wrote:

> This looks useful, but I am worried about the performance impact:
> 
> * The added allocation for every object that is finalized.
> * The event being enabled in the default configuration.
> 
> The default configuration must be safe to use even in pathological cases, i.e. an application with lots of objects being finalized. It's probably too much overhead (or number of events) for the profile configuration as well.
> 
> There is also the added startup cost of JFR. One event may not matter that much, but they all add up. We need to put in place a mechanism that doesn't rely on bytecode instrumentation at runtime. There is an enhancement for that, but it requires some engineering first.

I'm a bit worried by this too. Does it need to be enabled by default? Can we put a static final instance of FinalizerEvent in that class that can be used to test if the event is enabled so that it doesn't create a FinalizerEvent when disabled?

Is it worth exploring doing have an event in the VM, in register_finalizer, instead?

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PR: https://git.openjdk.java.net/jdk/pull/4101