There are problems with separating JVM and OS threads, mostly in assumptions that native library authors may have made about that mapping being 1:1. That isn't

There are problems with separating JVM and OS threads, mostly in assumptions that native library authors may have made about that mapping being 1:1. That isn't to say the situation can't be improved in specialised environments (see the JVM at Google talk at http://www.oracle.com/technetwork/java/javase/community/jlssessions-2255337.html http://www.oracle.com/technetwork/java/javase/community/jlss...) but fixing it in general is a hard problem. It is being thought about though - see John Rose's talk at this year's JVMLS.