Firstly, 1ms for a 3TB heap size is very unrealistic even for modern JVMs. Secondly, the figures that are often quoted for how long the program must be stopped

Firstly, 1ms for a 3TB heap size is very unrealistic even for modern JVMs. Secondly, the figures that are often quoted for how long the program must be stopped are for the "happy path" where enough memory can be freed that the program can continue without issue. In practice, what often happens with large heap sizes is that the GC starts lagging behind because there's not enough memory head-room. In these cases, the GC stops the world for much longer, because the program hangs on a memory allocation that cannot be satisfied until the GC completes. This primarily affects programs that have high rates of allocation. You can avoid this in Java with effort, but you end up with very non-idiomatic code.