On Madhu Konda's Weblog he writes:
Currently, our performance team takes care of "Middleware" product
performance including Glassfish, so check out our blogs from my team
members.
http://java.sun.com/docs/performance
http://java.sun.com/performance/reference/whitepapers/6_performance.html
http://java.sun.com/docs/hotspot/gc5.0/gc_tuning_5.html
http://java.sun.com/performance/reference/whitepapers/tuning.html
one more note: If you are on JDK 6, I highly recommend that you pick up jdk 6.0_14(update 14) since it has a improvements when using 64-bit large heaps by turning on Compressed OOPs (-XX:+UseCompressedOops when using -d64 switch).