Multiversion concurrency control for the generalized search tree

Many read-intensive systems where fast access to data is more important than the rate at which data can change make use of multidimensional index structures, like the generalized search tree (GiST). Although in these systems the indexed data are rarely updated and read access is highly concurrent, the existing coneurrency control mechanisms for multidimensional index structures are based on locking techniques, which cause significant overhead. In this article we present the multiversion-GiST (MVGiST), an in-memory mechanism that extends the GiST with multiversion concurrency control. The MVGiST enables lock-free read access and ensures a consistent view of the index structure throughout a reader's series of queries, by creating lightweight, read-only versions of the GiST that share unchanging nodes among themselves. An example of a system with high read to write ratio, where providing wait-free queries is of utmost importance, is a large-scale directory that indexes web services according to their input and output parameters. A performance evaluation shows that for low update rates, the MVGiST significantly improves scalability w.r.t. the number of concurrent read accesses when compared with a traditional, locking-based concurrency control mechanism. We propose a technique to control memory consumption and confirm through our evaluation that the MVGiST efficiently manages memory. Copyright (C) 2009 John Wiley & Sons, Ltd.