RFR: 8257774: G1: Trigger collect when free region count drops below threshold to prevent evacuation failures

[Aditya Mandaleeka]

On Tue, 23 Mar 2021 04:58:53 GMT, Aditya Mandaleeka <adityam at openjdk.org> wrote:

> _This PR picks up from [this previous PR](https://github.com/openjdk/jdk/pull/1650) by @charliegracie._
> 
> I won't repeat the full description from the prior PR, but the general idea is to add the notion of a "proactive GC" to G1 which gets triggered in the slow allocation path if the number of free regions drops below the amount that would be required to complete a GC if it happened at that moment. The threshold is based on the survival rates from eden and survivor spaces along with the space required for tenured space evacuations.
> 
> There are a couple of outstanding issues/questions known:
> - _Interaction with GCLocker_: In the case where we determine that a proactive GC is required and GC locker is active, we don't allow the young gen to expand (instead threads will stall). @tschatzl raised this and suggested that it should be discussed as part of the review.
> - _Disable proactive GC heuristic during initialization_: I ran into an issue in testing where code in the universe initialization codepath was tripping the proactive GC heuristic, leading to a GC being triggered before the VM has finished initialization. I need to find a good way to prevent this from happening. There may already be a mechanism for this, but I couldn't find one so I added a temporary placeholder (`zzinit_complete`) just to unblock testing.

There are one or two whitespace changes that I'll remove in a future commit prior to submission since the bot isn't happy about them. Will also handle the Windows build error.

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PR: https://git.openjdk.java.net/jdk/pull/3143