In https://github.com/openjdk/jdk/pull/28738 (JDK-8373403), TestG1ClassUnloadingHWM.java shows that with -XX:G1HeapRegionSize=32m or larger region sizes, G1 could trigger unnecessary concurrent cycles in the testWithoutG1ClassUnloading() case.

[~tschatzl] wrote: if there is only one region, there is no gain in collecting that region (unless, very unlikely, that single region is completely empty). Either the concurrent mark will select it as an evacuation candidate and copy its contents to another (single) region, or do nothing.

[~iwalulya] proposed a potential fix:
--- a/src/hotspot/share/gc/g1/g1IHOPControl.cpp
+++ b/src/hotspot/share/gc/g1/g1IHOPControl.cpp
@@ -115,7 +115,7 @@ size_t G1IHOPControl::get_conc_mark_start_threshold() {
   guarantee(_target_occupancy > 0, "Target occupancy must be initialized");
 
   if (!_is_adaptive || !have_enough_data_for_prediction()) {
- return (size_t)(_initial_ihop_percent * _target_occupancy / 100.0);
+ return MAX2((size_t)(_initial_ihop_percent * _target_occupancy / 100.0), G1HeapRegionSize);
   }