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Sorry, I'm using the modern meaning of green threads [0], which is simply threads scheduled in userspace. The first green threads implementation, which was of

Sorry, I'm using the modern meaning of green threads [0], which is simply threads scheduled in userspace. The first green threads implementation, which was of course Java 1.0, was certainly M:1 - my memory is that we didn't even have pthreads in linux at the time - but the term today doesn't specifically mean a M:1 mapping. On the other hand, of course JVM1.0 invented "green threads" which was M:1. So yeah this is confusing and I made it more confusing. Sorry. :) So the point I was trying to make is that Java originally scheduled threads in userspace, then delegated scheduling to the OS, and has now taken it back. In hindsight it seems that delegating this key function to the OS has caused a lot of pain, and I'm interested in the drivers for using OS threads instead of just making the original green thread implementation better. To be even clearer, I see this a lot in software. We delegate some key function of a product (like database access) to some inefficient and complex machinery (like an ORM) and then spend years working around the problems that decision caused. I've done this myself. I'd like to understand how to stop doing it. [0] https://en.wikipedia.org/wiki/Green_threads https://en.wikipedia.org/wiki/Green_threads

deleted 6y ago
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pron 6y ago
You are largely correct about the "reversion" to userspace threading, but the decisions (some of them were forced moves) weren't because of fashions but very different circumstances. You could point out that it is interesting that different circumstances have led to userspace threads. I don't know all the reasons behind abandoning green threads, but some are mentioned here: https://docs.oracle.com/cd/E19455-01/806-3461/6jck06gqe/index.html https://docs.oracle.com/cd/E19455-01/806-3461/6jck06gqe/inde... namely, no multi-core and bad FFI ("interoperate with existing MT applications"). Plus, a little later, the transition to HotSpot left little desire to reimplement userspace threading. There was no pressing need, there was other, higher-priority work needed, and FFI was still an impediment. In other words, making green threads better was neither an important requirement nor a reasonable possibility back then. Why have we "gone back"? 1. the scale of concurrency required today is higher and resulted in the scaffolding I mention in the article, 2. FFI is no longer an impediment as Java code rarely relies on native code these days. Nevertheless, Project Loom doesn't switch from kernel to userspace threads, but rather lets you choose which implementation you want for each thread.

deleted 6y ago

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pron 6y ago

You are largely correct about the "reversion" to userspace threading, but the decisions (some of them were forced moves) weren't because of fashions but very different circumstances. You could point out that it is interesting that different circumstances have led to userspace threads. I don't know all the reasons behind abandoning green threads, but some are mentioned here: https://docs.oracle.com/cd/E19455-01/806-3461/6jck06gqe/index.html https://docs.oracle.com/cd/E19455-01/806-3461/6jck06gqe/inde... namely, no multi-core and bad FFI ("interoperate with existing MT applications"). Plus, a little later, the transition to HotSpot left little desire to reimplement userspace threading. There was no pressing need, there was other, higher-priority work needed, and FFI was still an impediment. In other words, making green threads better was neither an important requirement nor a reasonable possibility back then. Why have we "gone back"? 1. the scale of concurrency required today is higher and resulted in the scaffolding I mention in the article, 2. FFI is no longer an impediment as Java code rarely relies on native code these days. Nevertheless, Project Loom doesn't switch from kernel to userspace threads, but rather lets you choose which implementation you want for each thread.