If it interrupts your running code, it only can happen at moments when you allocate memory, just like with malloc. You cannot predict when malloc calls brk(), either.

Also, most OS-es are non-deterministic, anyways. You cannot predict whether your code lives in cache/main memory, or on disk. In some systems, your data even might have to be brought in by a tape robot without your code being aware of it. As soon as a garbage collector manages to get its interruptions to be about equal to delays caused by caching and virtual memory, I doubt many people will care.