Remembered Set is one of the key constructs of G1, it keeps knowledge about cross region references.

Idea of a test checking that Remembered Set works good when growing up and down:

int[] crossRefs = {0, 100, 10, 200, 50, 300, 0};
for (int refN: crossRefs) {
    for (x: G1Regs) {
        for (y: G1Regs) {
            // set refN references from region X to region Y
            WB.concMarkCycle();
        }
    }
}