Chapter 3. Shenandoah garbage collector modes

You can run Shenandoah in three different modes. Select a specific mode with the -XX:ShenandoahGCMode=<name>. The following list describes each Shenandoah mode:

normal/satb (product, default)
This mode runs a concurrent garbage collector (GC) with Snapshot-At-The-Beginning (SATB) marking. This marking mode does the similar work as G1, the default garbage collector for Red Hat build of OpenJDK 21.
iu (experimental)
This mode runs a concurrent GC with Incremental Update (IU) marking. It can reclaim unreachably memory more aggressively. This marking mode mirrors the SATB mode. This may make marking less conservative, especially around accessing weak references.
passive (diagnostic)
This mode runs Stop the World Event GCs. This mode is used for functional testing, but sometimes it is useful for bisecting performance anomalies with GC barriers, or to ascertain the actual live data size in the application.