Pauseless [0]? It originally used hardware support on Azul machines to trap upon reading an unmarked pointer, but now (in the C4 generational incarnation, as well as in ZGC) this is done in software. But the other collectors use snapshot-at-the-beginning and a write barrier for concurrent marking, which originated in 1990 [1].
[0] https://www.usenix.net/legacy/events/vee05/full_papers/p46-c...