Green Threads vs Non Green Threads

The Wikipedia article Green Threads explains it very well.

Green threads are "user-level threads". They are scheduled by an "ordinary" user-level process, not by the kernel. So they can be used to simulate multi-threading on platforms that don't provide that capability.

In the context of Java specifically, green threads are a thing of the past. See article JDK 1.1 for Solaris Developer's Guide. (It's about Solaris, but the fact that green threads are not used anymore is valid for the usual platforms).

Green threads were abandoned in the Sun JVM for Linux as of the release of version 1.3 (see Java[tm] Technology on the Linux Platform on archive.org). That dates back to 2000. For Solaris, native threads were available from JDK 1.2. That dates back to 1998. I don't even think there ever was a green thread implementation for Windows, but I can't find a reference for that.

There are some exceptions as noted in the Wikipedia article, I gather mostly for low-power (embedded) devices.

Green threads are threads implemented at the application level rather than in the OS. This is usually done when the OS does not provide a thread API, or it doesn't work the way you need.

Thus, the advantage is that you get thread-like functionality at all. The disadvantage is that green threads can't actually use multiple cores.

There were a few early JVMs that used green threads (IIRC the Blackdown JVM port to Linux did), but nowadays all mainstream JVMs use real threads. There may be some embedded JVMs that still use green threads.

Green thread memory is allocated from the heap rather than having a stack created for it by the OS. This can potentially give an order of magnitude or more increase in concurrent threads. As other people have mentioned, this would not take advantage of multiple processors automatically, however the use case is typically for blocking I/O -- for example green threads might allow you to handle 100k concurrent connections as opposed to 10k.

So in other words, green threads are better for IO bound operations at a certain scale.