perf: add direct-mapped node cache to BTreeMap#416
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Add a 32-slot direct-mapped node cache to BTreeMap that avoids re-loading hot nodes from stable memory. Modeled after CPU caches: O(1) lookup via (address / page_size) % 32, collision = eviction. Read paths (get, contains_key, first/last_key_value) use a take+return pattern to borrow nodes from the cache without RefCell lifetime issues. Write paths (insert, remove, split, merge) invalidate affected cache slots. Key changes: - Switch get() from destructive extract_entry_at to node.value() - Remove unused extract_entry_at method - Change traverse() closure from Fn(&mut Node) to Fn(&Node) - Invalidate cache in save_node, deallocate_node, merge, clear_new Expected improvement: ~15-20% for random reads, ~65% for hot-key workloads, ~0% overhead for writes (cache.get_mut() bypasses RefCell).
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Summary
(address / page_size) % 32, collision = eviction (no LRU tracking)get,contains_key,first/last_key_value) use a take+return pattern to avoid re-loading hot upper-tree nodes from stable memorysave_node,deallocate_node,merge, andclear_newget()from destructiveextract_entry_at(swap_remove) to non-destructivenode.value()(borrows via OnceCell)extract_entry_atmethodThis subsumes all four previous caching approaches (root-only, LRU+clone, LRU+Rc, page cache) into a single design that:
Node<K>directly (no Rc, no Clone, no heap allocation per cache entry)cache.get_mut()on write paths (zero RefCell overhead)Expected improvement: ~15-20% for random reads, ~65% for hot-key workloads, ~0% overhead for writes.