A GC cycle of ZGC includes multiple phases:

• Pause Mark Start
• Pause Mark End
• Pause Relocate Start

In the first phase, Pause Mark Start, ZGC marks objects that have been pointed to by roots. This includes walking through the live set of objects, and then finding and marking them. This is by far one of the most heavy-duty workloads in the ZGC GC cycle.

Once this completes, the next cycle is Pause Mark Start, which is used for synchronization and starts with a short pause of 1 ms. In this second phase, ZGC starts with reference processing and moves to week-root cleaning. It also includes the relocation set selection. ZGC marks the regions it wants to compact.

The next step, Pause Relocate Start, triggers the actual region compaction. It begins with root scanning pointing into the location set, followed by the concurrent reallocation of objects in the relocation set.

The first phase, that is, Pause Mark Start, also includes remapping the live data. Since marking and remap of live data is the most heavy-duty GC operation, it isn't executed as a separate one. Remap starts after Pause Relocate Start but overlaps with the Pause Mark Start phase of the next GC cycle.