RFR: 8284161: Implementation of Virtual Threads (Preview) [v3]

[Jaikiran Pai]

On Fri, 15 Apr 2022 11:45:01 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> This is the implementation of JEP 425: Virtual Threads (Preview); TBD which JDK version to target.
>> 
>> We will refresh this PR periodically to pick up changes and fixes from the loom repo.
>> 
>> Most of the new mechanisms in the HotSpot VM are disabled by default and require running with `--enable-preview` to enable.
>> 
>> The patch has support for x64 and aarch64 on the usual operating systems (Linux, macOS, and Windows). There are stubs (calling Unimplemented) for zero and some of the other ports. Additional ports can be contributed via PRs against the fibers branch in the loom repo.
>> 
>> There are changes in many areas. To reduce notifications/mails, the labels have been trimmed down for now to hotspot, serviceability and core-libs. We'll add the complete set of labels when the PR is further along.
>> 
>> The changes include a refresh of java.util.concurrent and ForkJoinPool from Doug Lea's CVS. These changes will probably be proposed and integrated in advance of this PR.
>> 
>> The changes include some non-exposed and low-level infrastructure to support the (in draft) JEPs for Structured Concurrency and Scope Locals. This is to make life a bit easier and avoid having to separate VM changes and juggle branches at this time.
>
> Alan Bateman has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
> 
>  - Refresh
>  - Refresh
>  - Merge with jdk-19+18
>  - Refresh
>  - Initial push

src/java.base/share/classes/java/lang/Thread.java line 1554:

> 1552:      * Null out reference after Thread termination (JDK-4006245)
> 1553:      */
> 1554:     void clearReferences() {

I had a look at the referenced [JDK-4006245](https://bugs.openjdk.java.net/browse/JDK-4006245). The previous/existing reference cleaning logic seems to have been added mainly to `null` the reference to the user/application class instance represented by the `Runnable` task. Of course, other references too were being cleared. The description in the linked JBS seems to suggest that the `Thread` instance wasn't being GCed sooner. That was more than 2 decades back. I don't have enough knowledge of this area to know if this is still an issue in recent GC implementations. 

With the new implementation in this PR, this `clearReferences()` will no longer set the user/application `task` instance to `null`. I understand why we aren't doing that - the `task` field in the `FieldHolder` is `final` (IMO, rightly so). This, in theory, undoes what JDK-4006245 had implemented. I don't know if we still need such a change here and I'm just bringing this up to see if anyone else has any opinions or if any additional experiments need to be run in context of this change.

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PR: https://git.openjdk.java.net/jdk/pull/8166