You have two kind of concurrent GCs, the ones where the marking phase in concurrent with the application and the ones where the evacuation phase is also concurr

You have two kind of concurrent GCs, the ones where the marking phase in concurrent with the application and the ones where the evacuation phase is also concurrent with the application. G1 only does the marking concurrently, the evacuation is done in small pauses. A decade ago, there was only one concurrent evacuation GC available in Java, C4 from Azul. Now, we have Shenandoah and ZGC.