From what I understand the goals are similar but the approaches are different. - Shenandoah uses Brooks-style forward pointers whereas ZGC uses colored pointer

From what I understand the goals are similar but the approaches are different. - Shenandoah uses Brooks-style forward pointers whereas ZGC uses colored pointers and off-heap forwarding tables. - Shenandoah could in theory run on Windows, AFAIK this is a non-goal for ZGC. - Shenandoah tries to return unused heap to the OS wheres currently the recommendation for ZGC seems to be -Xms == -Xmx. In addition ZGC tripple maps the heap which can lead to interesting challenges in resource usage accounting. - None of them are generational although Shenandoah allows for custom policies. - Both of them seem to disable biased locking by default. I would guess the latency for deoptimizing is simply too large. - Shenandoah supports pinning objects in JNI criticals without disabling the GC. - Somewhat unsurprisingly ZGC introduces several HotSpot latency optimizations that will also benefit Shenandoah (thread-local handshakes, concurrent reference processing, ...).