Reiser4, Part II: Designing Trees that Cache Well
This article is the second in a series on the design of the Reiser4 filesystem. The first article [LJ, December 2002] defined basic concepts: trees, nodes and items. This article explains why balanced trees are better than unbalanced trees and why B+trees are better than B-trees by explaining and applying the principles of caching. The article then applies these same principles to a classic database technique used in ReiserFS v3 called binary large objects (BLOBs). It suggests that BLOBs reduce the effectiveness of caching internal nodes by making the tree no longer truly balanced. It also shows how Reiser4 stores objects larger than a node without unbalancing the tree.
I apologize to readers for the delay of this article, which is due to the Halloween feature-freeze for 2.6 and the need to stabilize Reiser4 quickly at that time.
The fanout rate (n) refers to the number of nodes pointed to by each level's nodes (Figure 1). If each node can point to n nodes of the level below it, then starting from the top, the root node points to n internal nodes at the next level, each of which points to n more internal nodes at its next level and so on. m levels of internal nodes can point to nm leaf nodes containing items in the last level. The more you want to store in the tree, the larger you have to make the fields in the key that first distinguish the objects, then select parts of the object (the offsets). This means your keys must be larger, which decreases fanout (unless you compress your keys, but that will wait for our next version).
In Figure 1, the first graph is a four-level tree with a fanout of n = 1. It has only four nodes, starting with the (red) root node, traversing the (burgundy) internal and (blue) twig nodes and ending with the (green) leaf node, which contains the data. The second tree, with four levels and a fanout of n = 2, starts with a root node, traverses two internal nodes, each of which points to two twig nodes (for a total of four twig nodes) and each of these points to two leaf nodes for a total of eight leaf nodes. Lastly, a four-level, fanout of n = 3 tree is shown, which has one root node, three internal nodes, nine twig nodes and 27 leaf nodes.
You can store not only pointers and keys in internal nodes but also the objects to which those keys correspond. This is what the original B-tree algorithms did (Figure 2).
Then, B+trees were invented that have only pointers and keys stored in internal nodes with all of the objects stored at the leaf level (Figure 3).
Fanout is increased when we put only pointers and keys in internal nodes and don't dilute them with object data. Increased fanout raises our ability to cache all of the internal nodes, because there are fewer internal nodes. People often respond to this by saying, “but B-trees cache objects, and caching objects is just as valuable.” This is not, on average, the answer. Of course, discussing averages makes the discussion much more difficult.
However, we need to cover some cache design principles before getting to this. Let's suppose the following:
You have two sets of things, A and B.
You need things from those two sets semi-randomly, with a tendency for some things to be needed much more frequently than others, but which things those are can shift over time.
You can keep things around after you use them in a cache of limited size.
You tie the caching of each thing from A to the caching of some particular thing from B. This means that whenever you fetch something from A into the cache, you fetch its partner from B into the cache.
This increases the amount of cache required to store everything recently accessed from A. If there is a strong correlation between the need for the two particular objects that are tied in each of the pairings, stronger than the gain from spending those cache resources on caching more members of A and B according to the LRU (least recently used) algorithm, then this might be worthwhile. If no such strong correlation exists, it is bad. LRU means that we choose the least recently used thing to discard from the cache when we need to make more room. Various approximations of LRU are the most commonly used caching algorithms in OS design.
But wait, you might say, you need things from B also, so it is good that some of them were cached. Yes, you need some random subset of B. The problem is that without a correlation, the things from B that you need are not especially likely to be those same things from B that were tied to the things from A that were needed. Choosing what from B you bring into the cache and keep in the cache on the basis of something other than LRU may reduce the effectiveness of caching, unless it is done according to an algorithm at least as good as LRU. Often choosing which members of B to cache based on which members of A have been cached is not as good as LRU, and so we have a problem.
This tendency to inefficiently tie things that are randomly needed exists outside the computer industry. For instance, suppose you like both popcorn and sushi, with your need for them on a particular day being random. Suppose that you like movies randomly. Suppose a theater requires you to eat only popcorn while watching the movie you randomly found optimal to watch, and not eat sushi from the restaurant on the corner while watching that movie. Is this a socially optimum system? Suppose quality is randomly distributed across all hot dog vendors. If you can only eat the hot dog produced by the best movie displayer on a particular night that you want to watch a movie, and you aren't allowed to bring in hot dogs from outside the movie theater, is this a socially optimum system? Optimal for you?
Tying strongly correlated things together can sometimes be good for performance, however. Many filesystems tie access to information about the file's size to information about the file's name. This seems to work well, better than LRU would.
Tying uncorrelated things together is a common error in designing caches but is still not enough to describe why B+trees are better. With internal nodes, we store more than one pointer per node, meaning pointers are not cached separately. You could argue that pointers and the objects to which they point are more strongly correlated than the different pointers. I hope what we have discussed here is instructive, but we still need another cache design principle.
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