BP-Wrapper: A System Framework Making Any Replacement Algorithms (Almost) Lock Contention Free

In a high-end database system, the execution concurrency level rises continuously in a multiprocessor environment due to the increase in number of concurrent transactions and the introduction of multi-core processors. A new challenge for buffer management to address is to retain its scalability in responding to the highly concurrent data processing demands and environment. The page replacement algorithm, a major component in the buffer management, can seriously degrade the system's performance if the algorithm is not implemented in a scalable way. A lock-protected data structure is used in most replacement algorithms, where high contention is caused by concurrent accesses. A common practice is to modify a replacement algorithm to reduce the contention, such as to approximate the LRU replacement with the clock algorithm. Unfortunately, this type of modification usually hurts hit ratios of original algorithms. This problem may not exist or can be tolerated in an environment of low concurrency, thus has not been given enough attention for a long time. In this paper, instead of making a trade-off between the high hit ratio of a replacement algorithm and the low lock contention of its approximation, we propose a system framework, called BP-Wrapper, that (almost) eliminates lock contention for any replacement algorithm without requiring any changes to the algorithm. In BP-Wrapper, we use batching and prefetching techniques to reduce lock contention and to retain high hit ratio. The implementation of BP-Wrapper in PostgreSQL version 8.2 adds only about 300 lines of C code. It can increase the throughput up to two folds compared with the replacement algorithms with lock contention when running TPC-C-like and TPC-W-like workloads.