I'm not confident enough to answer your question straight away, but I did
some reading on linux threading and JVMs before we built one production server.
Linux added NPTL (Native POSIX Thread Library) from version 2.6.x. This thread library is a much better implementation than the previous thread lib. You get NPTL in Redhat 9.x- (2.4.x + patches).
To be able to take advantage of the new thread library you also have to use J2SE 1.4.2-.
This combination above should give a lot better thread scalability,
especially fo a J2EE server (lot of threads).
You will find more info on linux + JVM threads at