There are few problems with current implementation of InVM courier.
1) The message queue uses ConcurrentLinkedQueue as backing data structure. Unfortunately the synchronized primitive is used as lock on this queue whicm means that all advanatages of using it are negated. I guess that this approch is used for lockstep. The correct soultion is should be to have messageQueue unsychchronized and have separate Event object or separate regular object as lock holder if lockstep is in place.
2) The lockstep does not work correctly when there are multiple threads on input/output of the queue. The notify call does not guarantee that the correct delivery/pickup threads are waken up. For example the notify call in one delivery thread can wake up another delivery thread waiting up for the notify from pickup thread.