RFR: 8212879: Make JVMTI TagMap table concurrent [v4]

[Coleen Phillimore]

On Thu, 5 Nov 2020 14:36:44 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> Ok, so there were many test failures with other approaches.  Having GC trigger the posting was the most reliable way to post the events when the tests (and presumably the jvmti customers) expected the events to be posted.  We could revisit during event disabling if a customer complains about GC pause times.
>
> The point of this change was not necessarily to be lazy about updating the tagmap, until someone uses it. The point was to get rid of the last annoying serial GC phase. Doing it all lazily would certainly also achieve that. But it would also lead to situations where no event is ever posted from GC to GC. So you would get the event 20 GCs later, which might come as a surprise. It did come as a surprise to some tests, so it is reasonable to assume it would come as a surprise to users too. And I don't think we want such surprises unless we couldn't deal with them. And we can.

Kim's change to post the events from the service thread or before other JVMTI operations removes posting events from  the gc_notification, which was the objection.

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PR: https://git.openjdk.java.net/jdk/pull/967