When talking about in-memory search tree, we usually think of various binary search trees: red-black tree, AVL tree, treap, splay tree and so on. We do not often think of B-tree, as B-tree is commonly introduced as an on-disk data structure rather than in-memory one. Is B-tree also a good data structure for in-memory ordered dictionary? I used to search for the performance comparison between B-tree and binary search trees, but ended up with nothing useful. It seems that only I am interested in such comparison and so I have to do it by myself.

I found John-Mark Gurney’s B-tree via google search. It is well coded and full of clever ideas. The original version has small memory footprint, but it is not as fast as STL’s red-black tree. I studied this source codes and think I should be able to further optimize it. In the end, I got my kbtree.h macro library. As you can see in my hash table benchmark, the modified version beats STL set while using even smaller memory than the original version. I think I am now at the position to say: at least for some applications, B-tree is a better ordered data structure than most of binary search trees.

The most attractive feature of B-tree is its small memory usage. A binary tree needs at least two pointers for each record, which amounts to 16N on a modern 64-bit systems. A B-tree only needs one pointer. Although in a B-tree each node may not be full, a sufficiently large B-tree should be at least 50% full by definition and in average around 75% full. On a 64-bit system, the extra memory is only 8N/0.75+KN(1/0.75-1)=(10+0.3K)N, where K is the size of a key. In fact we can do even better as we do not need to allocate the null pointers in leaves. The practical memory overhead can be reduced to below (5+0.3K)N (in fact, as the majority of nodes in a B-tree are leaves, the factor 5 should be smaller in practice), far better than a binary search tree. On speed, no binary search tree with just two additional pointers (splay tree and hash treap) can achieve the best performance. We usually need additional information at each node (AVL tree and standard red-black tree) or a random number (treap) to get good performance. B-tree is different. It is even faster than the standard red-black tree while still using (5+0.3K)N extra memory! People should definitely pay more attention to B-tree.

Update: The modified B-tree is available here (HTML) as a single C header file. Example is here. Currently, the APIs are not very friendly but are ready to use. In case you want to give a try. Note that you must make sure the key is absent in the tree before kb_put() and make sure the key is present in the tree before calling kb_del().

Someone has corrected me. STL is a specification, not an implementation. By STL in my blog, I always mean SGI STL, the default STL that comes with GCC.

Over the weekend I have done a more complete benchmark of various libraries on search trees and hash tables. Please read this post if you are interested.

I realize that a lot of red-black tree implementations do not need a parent pointer, although SGI STL’s one uses. My comment below is somewhat wrong.

Update 2: kbtree.h has long been moved here, along with khash.h and my other 1- or 2-file libraries.