RFR: 8275338: Add JFR events for notable serialization situations [v2]

[Raffaello Giulietti]

On Sat, 16 Dec 2023 01:27:17 GMT, Erik Gahlin <egahlin at openjdk.org> wrote:

>> Raffaello Giulietti has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Restrict accessibility of nested classes.
>
> It seems correct to have the event disabled in both configurations. It's not hard to imagine pathological cases where millions of stream objects are created, which would create overhead and flood event buffers, making JFR not usable.
> 
> That said, I wonder how many will actually use, or find out about the event, if they need to edit the .jfc file or specify -XX:StartFlightRecording:jdk.SerializationMisdeclaration#enabled=true on command line? 
> 
> How important is the stack trace when diagnosing serialization misdeclarations?
> 
> We faced a similar situation with the Finalization event a couple of years ago, but we were able to make the event periodic and enabled by default with some VM magic. An event that is enabled by default will probably be seen by 10 000 times as many users, so it might be worth considering other approaches, even if they are harder to implement, or we lose some of the information.

@egahlin Yes, disabled by default means less overhead in the usual case.

On the other hand, the checks and the events generation are done for serializable classes that actually actively participate in serialization, lazily, and just once per class, so maybe they are not as invasive and might be "always enabled". Pathological cases with millions of serializable classes might indeed lead to problems with "always enable", although they should be quite rare.

I don't think that stack traces are that useful for this kind of event, since they are mostly related to the static class structure rather than to runtime characteristics. Looking at the event itself should help in locating the problem quickly: it reports the class, the member, and what's questionable with it.

Or am I misunderstanding your points?

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PR Comment: https://git.openjdk.org/jdk/pull/17129#issuecomment-1860655448