RFR: 8331553: Windows JVM leaks Event and Thread handles when multiple threads are used [v2]

[Daniel Jeliński]

On Wed, 19 Jun 2024 16:50:22 GMT, Daniel Jeliński <djelinski at openjdk.org> wrote:

>> We use 2 ParkEvent instances per thread. The ParkEvent objects are never freed, but they are recycled when a thread dies, so the number of live ParkEvent instances is proportional to the maximum number of threads that were live at any time.
>> 
>> On Windows, the ParkEvent object wraps a kernel Event object. Kernel objects are a limited and costly resource. In this PR, I replace the use of kernel events with user-space synchronization.
>> 
>> The new implementation uses WaitOnAddress and WakeByAddressSingle methods to implement synchronization. The methods are available since Windows 8. We only support Windows 10 and newer, so OS support should not be a problem.
>> 
>> WaitOnAddress was observed to return spuriously, so I added the necessary code to recalculate the timeout and continue waiting.
>> 
>> Tier1-5 tests passed. Performance tests were... inconclusive. For example, `ThreadOnSpinWaitProducerConsumer` reported 30% better results, while `LockUnlock.testContendedLock` results were 50% worse. 
>> 
>> Thoughts?
>
> Daniel Jeliński has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update comment

I don't fully understand it either, but:
- the table is in user space, not in the kernel
- the entries are only inserted when a wait is active, and are removed when the wait is done
- when there's a race in waking up the thread, the kernel remembers that the thread needs to be woken up immediately the next time it waits; I suppose that's a flag in the thread structure, no extra memory required, but couldn't find a definitive answer in the docs.

Sources:
- Windows internals book https://www.oreilly.com/library/view/windows-internals-part/9780135462348/ch08.xhtml
- Raymond Chen's blog: https://devblogs.microsoft.com/oldnewthing/20160826-00/?p=94185

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PR Comment: https://git.openjdk.org/jdk/pull/19778#issuecomment-2182234703