First, there are no Java GCs that completely eliminate stop-the-world pauses. ZGC and Shenandoah reduce them to very short, sub-millisecond windows — but they still exist. Even the most concurrent collectors require STW phases for things like root scanning, final marking, or safepoint synchronization. This is documented in OpenJDK sources, benchmarks, and even in Oracle’s own whitepapers. Claiming Java has truly pause-less GC is simply false.

Second, you’re suggesting there are moving GCs that don’t use read barriers and don’t stop mutator threads at all. That’s technically implausible. Moving collectors by definition relocate objects, and unless you stop the world or have some read barrier/hazard indirection, you can’t guarantee pointer correctness during concurrent access. You must synchronize with the mutator somehow — either via stop-the-world, read barriers, or epoch/hazard-based coordination. It’s not magic, it’s basic memory consistency.

SGCL works without moving anything. That’s why it doesn’t need synchronization, read barriers, or even slow-path allocation stalls. That’s not a limitation — that’s a design goal. You can dislike the model, but let’s keep the facts straight.