Erlang has a really nice solution here by using a separate heap per Erland "process", and because they're owned by that process, can do a copying/compacting GC without having to take any kind of locking. Ruby's Guilds could adopt a similar idea and avoid STW GC there too.
Again, the term "stop the world" is misleading, there are a variety of compaction strategies that greatly minimize locking. There are even realtime GC implementations for the JVM.
Erlang's solution is interesting, but remember in Erlang processes can't share memory, they only have message passing. So you never have to worry about multiple threads writing to the same data structure at once, which is what makes compaction hard.
