For `G1BarrierSet::_dirty_card_queue_buffer_allocator`, in redirty_logged_cards, after merging cards into the barrier set’s dcq set, one can consider reduce the length of the underlying free list.

As part of that, need to figure out what data needs to be tracked for reduction heuristics. One value is the minimum _free_count. When allocating, have an additional member (_free_min) that tracks the minimum value of _free_count.
Note that reduction increases the risk of buffer allocation failure at some inopportune time later.

The removed `reduce_free_list` in JDK-8286387 might provide much inspiration, even if the code can't be used verbatim.