I think you may be confusing two different concepts here, which is understandable given the fact that this happened some 23 years ago.
The first versions of Java only had green threads. That was a cooperative threading model where only a single OS thread was used and the VM did a context switch while blocking for IO (in essence, every method declared as throwing InterruptedException could potentially context switch).
The mixed model you were talking about is most likely the threading model of Solaris where threads were split into a userspace part and a kernel part. It was possible to configure pthreads so as to use a multiple userspace threads on top of a single kernel thread (in a similar way as green threads worked in Java). It was also possible to run it in a one-to-one configuration or a version where you had a large number of userspace threads mapped to a smaller number of kernel threads. The last model was default in Solaris for a long time.
Eventually it was discovered that kernel threads were so fast in Solaris that there was practically never any benefit to the thread-pool model, and by Solaris 2.6 (I think, it's been a while so I could be wrong about the version) they changed it so that the default thread model was one-to-one.
The first versions of Java only had green threads. That was a cooperative threading model where only a single OS thread was used and the VM did a context switch while blocking for IO (in essence, every method declared as throwing InterruptedException could potentially context switch).
The mixed model you were talking about is most likely the threading model of Solaris where threads were split into a userspace part and a kernel part. It was possible to configure pthreads so as to use a multiple userspace threads on top of a single kernel thread (in a similar way as green threads worked in Java). It was also possible to run it in a one-to-one configuration or a version where you had a large number of userspace threads mapped to a smaller number of kernel threads. The last model was default in Solaris for a long time.
Eventually it was discovered that kernel threads were so fast in Solaris that there was practically never any benefit to the thread-pool model, and by Solaris 2.6 (I think, it's been a while so I could be wrong about the version) they changed it so that the default thread model was one-to-one.