IIRC Azul's pauseless GC uses kernel-level memory barriers to catch accesses to recently moved objects.

Azul still have pauses (10-100ms).

> Azul Systems has built a custom system (CPU, chip, board, and OS) specifically to run garbage collected virtual machines. The custom CPU includes a read barrier instruction. The read barrier enables a highly concurrent, parallel and compacting GC algorithm. The Pauseless GC algorithm is simple, efficient (low mutator overhead), and has no Stop-The-World pauses.

Azul is very similar to Shenandoah we are talking about. Updating of a wrong pointer still needs time, which can be unlucky when write trap is triggered in the middle of moving of large object.

It's the "stop-the-world" pauses that are problematic and that we've been talking about. Single-thread pauses are a fact of life, completely unavoidable with any dynamic memory allocation system even without GC (e.g. when malloc searches the free-lists).