Also, I remember I used to see separate entries for each thread in top on Linux. Not sure if this is still the case or if ps does the same thing.

For those of you who wonder why your JVM in Linux spawns so many processes:

"Unlike some other implementations of Unix (such as AIX or Solaris), the Linux kernel itself does not provide an abstraction of allowing several kernel threads to be packaged inside a process. Instead, a special version of the fork() system call (also known as clone()) has been implemented that allows you to specify that the child process should execute in the same address space as its parent. This system call is then used by the Linux threading library to present a thread programming abstraction to the user. From the user interface standpoint, practically all of the POSIX thread interfaces are fully supported under Linux; however, the implementation is not fully POSIX-compliant due to some problems in signaling. The result, however, is that each Java thread in the IBM Java VM for Linux is implemented by a corresponding user process whose process id and status can be displayed using the Linux ps command."

http://www-106.ibm.com/developerworks/library/java2/

Updated 2002-12-10 by mirod: turned the URL into a real link

Mac OS X does the same thing. It spawns new Mach processes for each Java thread. It was my understanding that it was not a bad thing. But I don't know enough about Darwin to really know for sure.

The 2.6 Linux kernel will support 2 fully POSIX-compliant implementations of threading. The one which is likely to be the default maintains the one thread, one kernel thread situation. However the simplicity of this arrangement also helps make it scale very well. See Native POSIX Thread Library 0.2 Released and Native POSIX Thread Library 0.1 Achieves 100,000 Concurrent Threads for more detailed information.