You mean it calls pthread_create for every Ruby thread? That would mean threads are no longer green threads, since both of the widely used Pthread libraries on Linux (Linuxthreads on 2.4 versions of the kernel, and NTPL on 2.6) have a 1-to-1 mapping of POSIX threads to kernel threads.

correct. but they each take a global interpreter lock ensuring only one is executing at any given time.