[ww520]

I don't understand what you are trying to get at. I merely stated that the immutability contract for concurrent access at the language level is not violated when the underlying GC mutes the memory. "Stop the world" is the simple technique to ensure that consistency. Things at lower level change/mute all the times. Virtual memory makes memories appear to to be there but really aren't. Your object might got swapped out and brought back in underneath you. But all those changes at lower levels don't change the contract at higher level.

If your problem is with pauses during GC, well guess what computers pause all the times. Whenever you move your mouse or type on the keyboard, they generate interrupts that pause the whole CPU and it has to handle them. Those pauses don't seem to be a problem. If the small pauses in GC are acceptable in general usage, why not use them?

If you want hard guarantee, that's what realtime system is for.