How Card Tables Optimize Cross‑Generational GCRoot Scanning in Java HotSpot

During young‑generation GCRoot scanning, objects that are only referenced from the old generation (e.g., object D) may be missed, leading to potential errors if they are incorrectly reclaimed.

To avoid scanning the entire old generation, a Remember Set (memory set) can be introduced in the young generation to record pointers from the old generation to the young generation.

The Remember Set is implemented in HotSpot as a "card table" (CardTable), a byte array where each entry corresponds to a fixed‑size memory region (a card page, typically 512 bytes). When a cross‑generational reference appears, the corresponding card entry is set to 1.

During young‑generation GCRoot scanning, the GC queries the card table to find cards marked with 1, indicating the presence of cross‑generational references, and includes the referenced objects in the GCRoot set.

By using the card table, the GC avoids scanning the entire old generation; even if the old generation holds thousands of live objects, only a small subset of cards need to be examined, greatly improving collection efficiency.

HotSpot maintains the card table via write barriers: when the old generation references a young‑generation object, the corresponding card entry is set to 1.