After vmop_entry_final_mark() identifies regions of immediate garbage, the current Shenandoah implementation does in order:

  entry_thread_roots()
  entry_weak_refs()
  entry_weak_roots()
  entry_cleanup_early()

The immediate garbage is not reclaimed until entry_cleanup_early(). We may experience out-of-memory during entry_thread_roots(), entry_weak_refs(), or entry_weak_roots().

By performing entry_cleanup_early() before entry_thread_roots(), we reduce the likelihood of OOM that might cause unnecessary degenerated GC cycles.