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project-loom-comparison
A comparison of different methods for achieving scalable concurrency in Java
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Well,
https://github.com/python-greenlet/greenlet
has been available for quite some time in Python (gevent probably being its most used flavor).
Note that it it actually predates the async/await approach which was incorporated into the Python language (so, in Python it was implemented as a third-party library -- even async/await had an implementation based on Python 2 using yield and some decorators: https://pypi.org/project/trollius/).
There's no design conflict I'm aware of but you don't "just" implement virtual threads. The PR to do it in HotSpot is notoriously huge:
https://github.com/openjdk/jdk/pull/8166
1,140 files, (+98,553 −9,862 LOC) and those lines of code are mostly horribly fiddly low level assembly/compiler hacking. This is partly why it took years of development.
Most people don't realize this, especially because in the later stages of course many others contributed, but Loom is in some sense one man's journey. Before he worked at Oracle Ron Pressler spent years writing a library called Quasar which implemented fibers on top of the JVM using bytecode rewriting and some low level hackery with internal APIs. At some point he became available for hiring and Oracle brought him on board, as by that point he was not only an expert in fiber implementations but also the JVM. That was the genesis of Loom.
Something I've learned from following the intricacies of VM development is that what we get is very much a result of hidden human stories as well as technical decisions. Features happen or don't happen on the basis of who was available to be hired at the time, as much as cold calculations of performance impacts. In turn that depends heavily on the vagaries of personal lives. The skills needed to do this work aren't that easy to find on the open market and training takes a long time.
For other VM implementors to do this, and realistically only .NET has the sort of languages where it makes sense (JS doesn't), well, it'd take a long time even if they start today and there's no guarantee of success.