RFR: 8300727: java/awt/List/ListGarbageCollectionTest/AwtListGarbageCollectionTest.java failed with "List wasn't garbage collected" [v2]

[Alexey Ivanov]

On Mon, 6 Mar 2023 17:24:13 GMT, Alexey Ivanov <aivanov at openjdk.org> wrote:

>> The test has become unstable recently, there were quite a few failures, on Windows mostly. I was lucky enough to find a host where the test failed consistently.
>> 
>> I call `System.gc()` directly as suggested in comments to the bug. I used `PhantomReference` instead of `WeakReference`.
>> 
>> Now the test calls `System.gc()` in a loop and waits for the reference to be enqueued. In majority of cases, the test exits the loop at the second attempt.
>
> Alexey Ivanov has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Use jdk.test.lib.util.ForceGC library class to Force GC

Eventually, core-libs have their own helper method in [`jdk.test.lib.util.ForceGC`](https://github.com/openjdk/jdk/blob/master/test/lib/jdk/test/lib/util/ForceGC.java). I replaced my custom waiter with `ForceGC`.

I asked a question on [core-libs-dev](https://mail.openjdk.org/pipermail/core-libs-dev/2023-March/101520.html) and [hotspot-gc-dev](https://mail.openjdk.org/pipermail/hotspot-gc-dev/2023-March/041574.html). Two approaches were suggested: `ForceGC` and `WhiteBox`. I chose `ForceGC` because it is simpler and easier to use.

Stuart Marks provided [a detailed reply](https://mail.openjdk.org/pipermail/core-libs-dev/2023-March/101547.html) with a couple of points:

> “I'd strongly recommend using this [ForceGC] in preference to allocating a lot of memory in order to provoke OutOfMemoryError. That technique has historically been a cause of test flakiness, and it still is, as you've discovered.”

> “It's true that System.gc() may sometimes be ignored -- for instance if Epsilon GC is enabled -- but for practical purposes, on Hotspot using a standard collector, calling it will eventually cause garbage collection and reference processing.”

Thomas also advises against using OOME:

> “I recommend to not use OOME for anything unless in extremely specific situations (test OOME is triggered or something). At/after OOME the VM is in a very precarious state that can give unexpected VM bailouts.”

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The latest revision uses a *standard* helper `ForceGC` class which is used in a number of tests.

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PR: https://git.openjdk.org/jdk/pull/12594