RFR: 8256916: Add JFR event for OutOfMemoryError

[Markus Grönlund]

On Fri, 15 Jan 2021 03:05:40 GMT, Yasumasa Suenaga <ysuenaga at openjdk.org> wrote:

>> I don't have time to look at this now, I need to fix some high priority issues for JDK 16, but the problem is with C2. Several people have looked at this before.
>
>> I don't have time to look at this now, I need to fix some high priority issues for JDK 16, but the problem is with C2. Several people have looked at this before.
> 
> I agree with you the problem is also with C2, however current JFR implementation has also problem in exception events - they hook throwing exceptions lesser than unified logging. It should be fixed.
> 
> As you can see in this PR, to move throwing events into HotSpot makes big change, so I think it is worth to work now without solving the problem with C2.
> (We will be able to make lesser change when we want to solve the problem with C2 later.)
> 
> So I hope other JFR folks review this PR if you are busy.

As you say: "move throwing events into HotSpot makes big change" - I agree and Erik has already stated that many have tried already to make this kind of change, but attempts have faltered because the impact / disruptions are unclear. Do you fully understand the consequences of how it will change the way users work with exceptions in JFR? If C2 optimizations now start to remove exception sites that were previously reported?

I understand and appreciate the ambition, and I acknowledge the existing mechanisms has drawbacks (known for a long time) but reworking how exceptions are reported is a big project. Such a project should also involve exception throttling - and throttling might pose additional constraints / opportunities that need to be considered.

Modifying this subsystem is a sensitive undertaking and should solve the fundamental and already known problems before it is to be endorsed and for the disruption to end user motivated.

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PR: https://git.openjdk.java.net/jdk/pull/1403