JDK 1.1 for Solaris Developer's Guide

One-to-One Model

The one-to-one model (one user thread to one kernel thread) is among the earliest implementations of true multithreading. In this implementation, each user-level thread created by the application is known to the kernel, and all threads can access the kernel at the same time. The main problem with this model is that it places a restriction on you to be careful and frugal with threads, as each additional thread adds more "weight" to the process. Consequently, many implementations of this model, such as Windows NT and the OS/2 threads package, limit the number of threads supported on the system.

Figure 2-2 One-to-One Multithreading Model