Does Java still have a word in every object allocation for locking it? Adding a 64-bit pointer sounds terribly inefficient.
Did you know Objective-C does locks and retain counting without allocating any extra fields in objects?
On hotspot: There are two bits in the header of every object. This is enough for an object that's never been used as a contended lock, CAS operations on the header can be used to handle the locking and that's that. As soon as you actually block on it, a 'real' lock is created (you can't get around the need for a list of threads to wake up as the lock is released) and the header grows to accomodate it. The process is called 'monitor inflation'. At a later date this might be cleaned up by a 'monitor deflation'.
Shenandoah uses a forwarding pointer in each object, adding overhead but limiting the problem only to write barriers. Here is Christine commenting on Azul vs Shenandoah [2]
From the talk: average pause is 6-7ms, max is 15ms, and the talk is one year old.
She hints at further developments in a version 2 which would make it entirely pauseless.
She has made another talk at RedHat's DevNation conference a few days ago, but they just won't put the video online arg!
[1] http://openjdk.java.net/jeps/189
[2] https://youtu.be/4d9-FQZZoVA?t=13m11s