RFR: 8287596: Reorg jdk.test.lib.util.ForceGC [v10]

[Daniel Fuchs]

On Thu, 30 Jun 2022 20:36:35 GMT, Roger Riggs <rriggs at openjdk.org> wrote:

>> I'm not sure if all unused object will be collected in one GC call always.  The gc() specification says "When control returns from the method call, the Java Virtual Machine has made a best effort to reclaim space from all unused objects. ... There is also no guarantee that this effort will determine the change of reachability in any particular number of objects, or that any particular number of {@link java.lang.ref.Reference Reference} objects will be cleared and enqueued."  But from the spec, I did not get a clear answer for the question.
>> 
>> If the `booleanSupplier` object could be cleared in a collection other than the `ref` collection, the current code may be safer.
>
> True, knowing when GC is 'done' is not deterministic except for a specify Reference to a specific object.
> System.gc is just a request, the checking for an object can more quickly exit the loop.
> The code is as is, and already commented, might wait an extra cycle, but only in the case of a race between the requested predicate and the object being reclaimed.  Ok as it.

Maybe not for this PR - but it could be useful to have a version of ForceGC that takes as parameter a ReferenceQueue<T> and a Reference<T>. That would be more efficient than creating a new object and waiting for a different cleaner thread to cleanup that object.

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PR: https://git.openjdk.org/jdk/pull/8979