I managed to capture 2 graphs taken during the load-testing sessions.
With embedded OpenDS in a 12-nodes OpenSSO environment, there will be consistent spikes happening at 15-20 mins interval and subsequently 25 mins ... (that was when I issued a thread dump and realized there were numerous threads related to OpenDS replication service)
Ideally, we would prefer the following graph. Consistently flat. Spikes at 15-min and 26-mins though, but minor enough to ignore. Still trying to further tune.
PS: Getting the almost flat graph above is not totally contributed by moving the embedded OpenDS to external Sun Directory Server. There were some other tuning made.