One of the problems with asking questions about JON is that not everyone has it because you have to pay for it. And the trial copy I had expired many months ago and was the 1.x version anyway.

In general, reaching zero free memory is not an issue. It just means that the heap is full and a garbage collection needs to take place. It also indicates to me that perhaps your young generation is not properly sized. You can read more about the heap and heap sizing at:

Documentation from Sun: http://java.sun.com/docs/hotspot/gc5.0/gc_tuning_5.html

A presentation I made: http://www.cecmg.de/doc/tagung_2007/agenda07/24-mai/2b3-peter-johnson/index.html

Things I would monitor (off the top of my head):

* number of active HTTP threads
* number of HTTP requests waiting for an available thread
* number of active database connections
* number of queued database connection requests (waiting for an available connection)
* amount of permgen space in use
* amount of heap space in use
* %CPU usage
* free RAM and/or system memory