Hotspot has several GC algorithms available, FYI. And the term "stop the world" is misleading, it doesn't fully capture the nuance of garbage collection. Any GC algorithm that does compaction will require some sort of blocking. You can do compaction incrementally, so you only lock small sections of the heap at a time, but that would still be considered "stop the world" despite being orders of magnitude faster than the naive case.

Go GC gets to make the claim that it doesn't "stop the world" because it doesn't do heap compaction. Which will become a major problem when dealing with large in-memory data structures or certain usage patterns.

And the fact that they still don't have a generational collector is concerning.